i965: Enable ARB_transform_feedback2 on Sandybridge.

The only feature over and above ES 3.0 is DrawTransformFeedback().

We already have to do the whole SOL_NUM_PRIMS_WRITTEN counter dance in
order to compute the SVBI value for ResumeTransformFeedback(), at which
point our existing GetTransformFeedbackVertexCount() implementation will
do the trick (though with a stall to CPU map the buffer).

Someday, we could probably implement DrawTransformFeedback() more
efficiently, using the "Load Internal Vertex Count" feature of
3DSTATE_SVB_INDEX and the 3DPRIMITIVE indirect vertex count bit.

Rumor has it this allows people to use WebGL 2.0 on Sandybridge.

Note that we don't need pipelined register writes like Gen7+ because
we use the 3DSTATE_SVB_INDEX command rather than MI_LOAD_REGISTER_MEM.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99842
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

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

@@ -483,6 +483,8 @@ brw_init_driver_functions(struct brw_context *brw,
       functions->EndTransformFeedback = brw_end_transform_feedback;
       functions->PauseTransformFeedback = brw_pause_transform_feedback;
       functions->ResumeTransformFeedback = brw_resume_transform_feedback;
+      functions->GetTransformFeedbackVertexCount =
+         brw_get_transform_feedback_vertex_count;
    }
 
    if (brw->gen >= 6)

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

@@ -158,6 +158,9 @@ intelInitExtensions(struct gl_context *ctx)
       ctx->Extensions.EXT_timer_query = true;
    }
 
+   if (brw->gen == 6)
+      ctx->Extensions.ARB_transform_feedback2 = true;
+
    if (brw->gen >= 6) {
       ctx->Extensions.ARB_blend_func_extended =
          !driQueryOptionb(&brw->optionCache, "disable_blend_func_extended");