i965/gs: Add GS_OPCODE_URB_WRITE.

This opcode will be used by geometry shaders to write output vertex
data to the URB.

It differs from VS_OPCODE_URB_WRITE in that it doesn't do an implied
move from R0 to the MRF holding the message header.  This will allow
the geometry shader to direct the output data to the appropriate
vertex in the output URB entry.
i965/gs: Add GS_OPCODE_THREAD_END.

This opcode will be used at the end of a geometry shader thread.  This
is different from what we do in vertex shaders because vertex shaders
always end by writing vertex data to the URB.  Geometry shaders, on
the other hand, write vertex data to the URB during normal execution,
so at the end of the thread, they simply have to output a count of how
many vertices were generated.

As with GS_OPCODE_URB_WRITE, this opcode doesn't do an implied move
from R0 to the MRF holding the message header.  This will allow the
vertex counts to be stored in the message header.
!UPSTREAM: i965/gs: Add GS_OPCODE_SET_WRITE_OFFSET.

TODO:
- Add more descriptive info in the commit message.
!UPSTREAM: i965/gs: Add GS_OPCODE_SET_VERTEX_COUNT.

TODO:
- Add more descriptive info in the commit message.
i965/gs: Add GS_OPCODE_SET_R0_2.

This opcode will be used by the geometry shader to initialize DWORD 2
of R0, which needs to be 0 in order for scratch reads and writes to
operate correctly.
!UPSTREAM: i965/gs: add GS visitors.
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.
!UPSTREAM: i965/gs: make the state atom for compiling Gen7 geometry shaders.

TODO: maybe disentangle
!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.
i965/gs(gen7): Allocate URB space for use by geometry shader.
!UPSTREAM: i965/gs(gen7): Allocate push constant space.
i965/vs: generalize push const upload in preparation for GS.
i965/gs: set up GS state.
!UPSTREAM: 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.

TODO:
- maybe squash with prev stuff
!UPSTREAM ===== END of preliminary back-end work =====
!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.
!UPSTREAM: Primitive ID part 1.
!UPSTREAM: Primitive ID part 2.
!UPSTREAM: Primitive ID part 3.