i965/gs(gen7): 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
(roughly) 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 =====
!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.
!UPSTREAM: Primitive ID part 1.
!UPSTREAM: Primitive ID part 2.
!UPSTREAM: Primitive ID part 3.
!UPSTREAM: Primitive ID part 4.
i965/gs: Fixup gl_PointSize on entry to geometry shaders.

gl_PointSize is stored in the w component of VARYING_SLOT_PSIZ, but
the geometry shader infrastructure assumes that it should look for all
geometry shader inputs of type float in the x component.  So when
compiling a geomtery shader that uses a gl_PointSize input, fix it up
during the shader prolog by moving the x component to the w component.

This is similar to how we emit fixups and workarounds for vertex
shader attributes.
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.
!UPSTREAM: i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

This will be used by geometry shaders to implement EndPrimitive()
functionality.

TODO:
- Move to appropriate place in series.
!UPSTREAM: i965/gs: implement EndPrimitive() functionality.

Stuff that needs fixing:
- Probably should pack vertex_count and cut_bits into a single register
- Code should be conditioned on whether the shader uses EndPrimitive()
!UPSTREAM: i965: Add new primitive types.

TODO: test.
!UPSTREAM: glsl: Add new geometry shader constants.

TOOD:
- These constants need to be plumbed through to the driver back-ends.
main: Expose geometry shader constants in GL 3.0.
!UPSTREAM: WIP: add gl_in to builtin_variables.cpp.
!UPSTREAM: mesa: fix transform feedback when a geometry shader is active.

TODO: test