intel/fs/gen12: Fix hangs with per-sample SIMD32 fragment shader dispatch.

intel/fs/gen12: Fix hangs with per-sample SIMD32 fragment shader dispatch.

The Gen12 docs are rather contradictory regarding the dispatch
configurations supported by the fragment shader -- The same table
present in previous generations seems to imply that only one dispatch
mode can be enabled when doing per-sample shading, but a restriction
documented in the 3DSTATE_PS_BODY page implies the opposite: That
SIMD32 can only be used in combination with some other dispatch mode.

The latter seems to match the behavior of real hardware as I could
tell from my testing: A bunch of multisample test-cases that do
per-sample shading hang if we only provide a SIMD32 shader.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>