i965: Reduce gl_MaxGeometryInputComponents to 64.

Although in principle there is no hardware limitation that prevents
gl_MaxGeometryInputComponents from being set to 128 on Gen7, we have
the following limitations in the vec4 compiler back end:

- Registers assigned to geometry shader inputs can't be spilled or
  later re-used for any other purpose.

- The last 16 registers are set aside for the "MRF hack", meaning they
  can only be used to send messages, and not for general purpose
  computation.

- Up to 32 registers may be reserved for push constants, even if there
  is sufficient register pressure to make this impractical.

A shader using 128 geometry input components, and having an input type
of triangles_adjacency, would use up:

- 1 register for r0 (which holds URB handles and various pieces of
  control information).

- 1 register for gl_PrimitiveID.

- 102 registers for geometry shader inputs (17 registers per input
  vertex, assuming DUAL_INSTANCED dispatch mode and allowing for one
  register of overhead for gl_Position and gl_PointSize, which are
  present in the URB map even if they are not used).

- Up to 32 registers for push constants.

- 16 registers for the "MRF hack".

That's a total of 152 registers, which is well over the 128 registers
the hardware supports.

Fortunately, the GLSL 1.50 spec allows us to reduce
gl_MaxGeometryInputComponents to 64.  Doing that frees up 48
registers, brining the total down to 104 registers, leaving 24
registers available to do computation.

Fixes piglit test
spec/glsl-1.50/execution/geometry/max-input-components.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_context.c

@@ -415,7 +415,7 @@ brw_initialize_context_constants(struct brw_context *brw)
    if (brw->gen >= 6) {
       ctx->Const.MaxVarying = 32;
       ctx->Const.VertexProgram.MaxOutputComponents = 128;
-      ctx->Const.GeometryProgram.MaxInputComponents = 128;
+      ctx->Const.GeometryProgram.MaxInputComponents = 64;
       ctx->Const.GeometryProgram.MaxOutputComponents = 128;
       ctx->Const.FragmentProgram.MaxInputComponents = 128;
    }