i965/msaa: Implement GL_SAMPLE_ALPHA_TO_{COVERAGE,ONE}.

This patch enables the multisampling parameters
GL_SAMPLE_ALPHA_TO_COVERAGE and GL_SAMPLE_ALPHA_TO_ONE, which allow
the fragment shader's alpha output to be converted into a sample
coverage mask and ignored for blending.  i965 supports these
parameters through the BLEND_STATE structure.

The GL spec allows, but does not require, the implementation to dither
the conversion from alpha to a sample coverage mask, so that alpha
values that aren't a multiple of 1/num_samples result in the correct
proportion of samples being lit.  A bit exists in the BLEND_STATE
structure to enable this functionality, but according to the hardware
docs it must be disabled on Sandy Bridge (see the Sandy Bridge PRM,
Vol2, Part1, p379: AlphaToCoverage Dither Enable).  So it is enabled
for Gen7 only.

Fixes piglit tests
"EXT_framebuffer_multisample/sample-alpha-to-{coverage,one} {2,4}".

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/drivers/dri/i965/gen6_cc.c

@@ -161,6 +161,13 @@ gen6_upload_blend_state(struct brw_context *brw)
       blend[b].blend1.write_disable_g = !ctx->Color.ColorMask[b][1];
       blend[b].blend1.write_disable_b = !ctx->Color.ColorMask[b][2];
       blend[b].blend1.write_disable_a = !ctx->Color.ColorMask[b][3];
+
+      /* _NEW_MULTISAMPLE */
+      blend[b].blend1.alpha_to_coverage =
+         ctx->Multisample._Enabled && ctx->Multisample.SampleAlphaToCoverage;
+      blend[b].blend1.alpha_to_one =
+         ctx->Multisample._Enabled && ctx->Multisample.SampleAlphaToOne;
+      blend[b].blend1.alpha_to_coverage_dither = (brw->intel.gen >= 7);
    }
 
    brw->state.dirty.cache |= CACHE_NEW_BLEND_STATE;
@@ -169,7 +176,8 @@ gen6_upload_blend_state(struct brw_context *brw)
 const struct brw_tracked_state gen6_blend_state = {
    .dirty = {
       .mesa = (_NEW_COLOR |
-	       _NEW_BUFFERS),
+               _NEW_BUFFERS |
+               _NEW_MULTISAMPLE),
       .brw = BRW_NEW_BATCH,
       .cache = 0,
    },