intel/fs/gen11: Work around dual-source blending hangs in combination with SIMD32.

intel/fs/gen11: Work around dual-source blending hangs in combination with SIMD32.

The SIMD8 dual-source blending framebuffer write messages seem to have
trouble releasing the pixel scoreboard dependency in SIMD32 dispatch
mode, which leads to hangs.  I have a better workaround for this which
doesn't involve disabling SIMD32 when dual-source blending is enabled,
but I'm still investigating some issues with it.  Limit the dispatch
width to SIMD16 in such cases for the moment in order to make the CI
happy on ICL with SIMD32 fragment shaders enabled.

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