i965: Implement transform feedback query support in hardware on Gen6+.

i965: Implement transform feedback query support in hardware on Gen6+.

Now that we have hardware contexts and can use MI_STORE_REGISTER_MEM,
we can use the GPU's pipeline statistics counters rather than going out
of our way to count primitives in software.

Aside from being simpler, this also paves the way for Geometry Shaders,
which can output an arbitrary number of primitives on the GPU.  It will
also allow us to use hardware primitive restart when these queries are
in use.

The GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN query is easy: it
corresponds to the SO_NUM_PRIMS_WRITTEN/SO_NUM_PRIMS_WRITTEN0_IVB
counters.

The GL_PRIMITIVES_GENERATED query is trickier.  Gen provides several
statistics registers which /almost/ match the semantics required:
- IA_PRIMITIVES_COUNT
  The number of primitives fetched by the VF or IA (input assembler).
  This undercounts when GS is enabled, as it can output many primitives.
- GS_PRIMITIVES_COUNT
  The number of primitives output by the GS.  Unfortunately, this
  doesn't increment unless the GS unit is actually enabled, and it
  usually isn't.
- SO_PRIM_STORAGE_NEEDED*_IVB
  The amount of space needed to write primitives output by transform
  feedback.  These naturally only work when transform feedback is on.
  We'd also have to add the counters for all four streams.
- CL_INVOCATION_COUNT
  The number of primitives processed by the clipper.  This doesn't work
  if the GS or SOL throw away primitives for rasterizer discard.
  However, it does increment even if the clipper is in REJECT_ALL mode.

Dynamically switching between counters would be painfully complicated,
especially since GS, rasterizer discard, and transform feedback can all
be switched on and off repeatedly during a single query.

The most usable counter is CL_INVOCATION_COUNT.  The previous two
patches reworked rasterizer discard support so that all primitives hit
the clipper, making this work.

v2: Occlusion query bug fixes removed and squashed in earlier patches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>