radv,aco: optimize computing the sample mask for per-sample shading

radv,aco: optimize computing the sample mask for per-sample shading

I don't know why these values were introduced for but it seems like
we can optimize this by just doing:

gl_SampleMaskIn[0] = (SampleCoverage & (1 << gl_SampleID))

AMDGPU-PRO and AMDVLK apply the same formula to compute the
sample mask when per-sample shading is enabled.

No fossils-db changes.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7377>