freedreno/a6xx: Fall back to masked RGBA blits for depth/stencil

freedreno/a6xx: Fall back to masked RGBA blits for depth/stencil

The blitter doesn't seem to have a write mask, so for depth only and
stencil only blits to Z24S8 we cast the Z24S8 buffer to an RGBA UNORM8
buffer and fall back to pipeline blits with corresponding write mask.

Fixes

  dEQP-GLES3.functional.fbo.blit.depth_stencil.depth24_stencil8_stencil_only
  dEQP-GLES3.functional.fbo.invalidate.sub.unbind_blit_depth
  dEQP-GLES3.functional.fbo.invalidate.sub.unbind_blit_msaa_depth
  dEQP-GLES3.functional.fbo.invalidate.whole.unbind_blit_depth
  dEQP-GLES3.functional.fbo.invalidate.whole.unbind_blit_msaa_depth
  dEQP-GLES3.functional.fbo.msaa.2_samples.stencil_index8
  dEQP-GLES3.functional.fbo.msaa.4_samples.stencil_index8

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Kristian H. Kristensen <hoegsberg@chromium.org>