i965/gs: add GS visitors.

This patch introduces the vec4_gs_visitor class, which translates
geometry shaders from GLSL IR to back-end opcodes.

This class is derived from vec4_visitor (which is also the base class
for vec4_vs_visitor), so as a result most of the back end code is
shared.  The only parts that differ are:

- Geometry shaders use a different input payload organization, since
  the inputs need to match up with the outputs of the previous
  pipeline stage (vec4_gs_visitor::setup_payload() and
  vec4_gs_visitor::setup_varying_inputs()).

- Geometry shader input array dereferences need a special stride
  computation, since all geometry shader inputs are interleaved into
  one giant array (vec4_gs_visitor::compute_array_stride()).

- There are no geometry shader system values
  (vec4_gs_visitor::make_reg_for_system_value()).

- At the beginning of a geometry shader, extra data in R0 needs to be
  zeroed out, and a vertex counter needs to be initialized
  (vec4_gs_visitor::emit_prolog()).

- When EmitVertex() appears in the shader, the current contents of
  output variables need to be emitted to the URB, and the vertex
  counter needs to be incremented
  (vec4_gs_visitor::visit(ir_emit_vertex *)).

- When generating a URB_WRITE message to output vertex data, the
  current state of the vertex counter needs to be used to store a
  write offset in the message header
  (vec4_gs_visitor::emit_urb_write_header()).

- The URB_WRITE message that outputs vertex data needs to be sent
  using GS_OPCODE_URB_WRITE, since VS_OPCODE_URB_WRITE would overwrite
  the offsets in the message header
  (vec4_gs_visitor::emit_urb_write_opcode()).

- At the end of a geometry shader, the final vertex count needs to be
  delivered using a URB WRITE message
  (vec4_gs_visitor::emit_thread_end()).

- EndPrimitive() functionality is not implemented yet
  (vec4_gs_visitor::visit(ir_end_primitive *)).

- There is no support for assembly shaders
  (vec4_gs_visitor::emit_program_code()).
i965: Make prim_to_hw_prim accessible outside brw_draw.c.

We will need access to this array in order to configure the geometry
shader.
i965/vec4: Make a function for setting up vec4 program key clip info.

This functionality will need to be reused by geometry shaders.
i965/gs: Add a data structure for tracking VS output VUE map.
i965/gs: Create brw_context::vec4_gs structure to track GS program state.
i965/gs: Support UBOs by generating surface state and binding tables.

All but two of the piglit GLSL 1.50/uniform_buffer tests work, and
maxuniformblocksize and referenced-by-shader work.
i965/gs: make the state atom for compiling Gen7 geometry shaders.
i965/gs: Allocate URB space for use by geometry shader.

Previously, we gave all of the URB space (other than the small amount
that is used for push constants) to the vertex shader.  However, when
a geometry shader is active, we need to divide it up between the
vertex and geometry shaders.

The size of the URB entries for the vertex and geometry shaders can
vary dramatically from one shader to the next.  So it doesn't make
sense to simply split the available space in two.  In particular:

- On Ivy Bridge GT1, this would not leave enough space for the worst
  case geometry shader, which requires 64k of URB space.

- Due to hardware-imposed limits on the maximum number of URB entries,
  sometimes a given shader stage will only be capable of using a small
  amount of URB space.  When this happens, it may make sense to
  allocate substantially less than half of the available space to that
  stage.

Our algorithm for dividing space between the two stages is to first
compute (a) the minimum amount of URB space that each stage needs in
order to function properly, and (b) the amount of additional URB space
that each stage "wants" (i.e. that it would be capable of making use
of).  If the total amount of space available is not enough to satisfy
needs + wants, then each stage's "wants" amount is scaled back by the
same factor in order to fit.

When only a vertex shader is active, this algorithm produces
equivalent results to the old algorithm (if the vertex shader stage
can make use of all the available URB space, we assign all the space
to it; if it can't, we let it use as much as it can).

In the future, when we need to support tessellation control and
tessellation evaluation pipeline stages, it should be straightforward
to expand this algorithm to cover them.
i965/gs: Allocate push constant space.

Previously, we would always use the same push constant allocation
regardless of what shader programs were being run: the available push
constant space was split into 2 equal size partitions, one for the
vertex shader, and one for the fragment shader.

Now that we are adding geometry shader support, we need to do
something smarter.  This patch adjusts things so that when a geometry
shader is in use, we split the available push constant space into 3
nearly-equal size partitions instead of 2.

Since the push constant allocation is now affected by GL state, it can
no longer be set up by brw_upload_initial_gpu_state(); instead it must
be set up by a state atom.
i965: Make sure constants re-sent after constant buffer reallocation.

The hardware requires that after constant buffers for a stage are
allocated using a 3DSTATE_PUSH_CONSTANT_ALLOC_{VS,HS,DS,GS,PS}
command, and prior to execution of a 3DPRIMITIVE, the corresponding
stage's constant buffers must be reprogrammed using a
3DSTATE_CONSTANT_{VS,HS,DS,GS,PS} command.

Previously we didn't need to worry about this, because we only
programmed 3DSTATE_PUSH_CONSTANT_ALLOC_{VS,HS,DS,GS,PS} once on
startup.  But now that we reallocate the constant buffers whenever
geometry shaders are switched on and off, we need to make sure the
constant buffers are reprogrammed.

We do this by adding a new bit, BRW_NEW_PUSH_CONSTANT_ALLOCATION, to
brw->state.dirty.brw.
i965/vs: generalize push const upload in preparation for GS.
i965/gs: set up GS state.
!UPSTREAM ===== END of preliminary back-end work =====
i965/gen7: Allow URB_WRITE channel masks to be used.

Previously, brw_urb_WRITE() would unconditionally override the channel
masks in the URB_WRITE message to 0xff (indicating that all channels
should be written to the URB).

For geometry shaders, we need the ability to set the channel masks
programatically, so create a flag to prevent brw_urb_WRITE() from
overriding them.
i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

This will be used by geometry shaders to implement EndPrimitive()
functionality.
i965/gs: Add opcodes needed for EndPrimitive().
i965/vec4: Add the ability to emit opcodes with just a dst register.

This is needed for GS_OPCODE_PREPARE_CHANNEL_MASKS.
!UPSTREAM: i965/gs: implement EndPrimitive() functionality.

Stuff that needs fixing:
- Code should be conditioned on whether the shader uses EndPrimitive()
!UPSTREAM: i965/gs: Generalize some error messages for GS.

TODO: find more places to generalize.
TODO: Don't use "VS/GS" terminology.  Use "vec4" terminology, since that will endure longer.
!UPSTREAM: Fix GS sampling.

TODO:

- Seems like these functions ought to be rewritten to loop over
  program types.

- Handling of null gs seems clumsy.

- Can the same sampler be used in multiple shader stages (e.g. both VS
  and FS)?  If not, then it seems like we're going to run out of
  samplers in our bitfield.  If so, then is it guaranteed that
  fs->SamplerUnits[s] == gs->SamplerUnits[s]?  If so this needs an
  explanatory comment.  If not, this code seems broken.

- Decide what to do about code duplication.  Either I should de-dup
  the code or I suppose I should only update the gen7 code.

- Move to appropriate place in the series.