intel/fs: Work around dual-source blending hangs in combination with SIMD16

intel/fs: Work around dual-source blending hangs in combination with SIMD16

It was found that dual-source blending hangs with SIMD16 dispatch in some
specific but unknown situation. Which in the wild happen when rgba
anti-aliasing is enabled for fonts.

Cc: <mesa-stable@lists.freedesktop.org>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2183
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5037>