freedreno/a6xx: combine sample mask into blend state

freedreno/a6xx: combine sample mask into blend state

This gets rid of one lone register we used to emit directly in IB2
whenever blend state changes, at the expense of needing blend state
variants when sample-mask changes.  I think typically sample-mask
should not change frequently, so this seems like a fair trade-off.

To further limit the # of variants, we ignore sample-mask bits that
are not relavant for the current # of samples.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4813>