radeonsi: apply a mask to gl_SampleMaskIn in the PS prolog

radeonsi: apply a mask to gl_SampleMaskIn in the PS prolog

gl_SampleMaskIn is supposed to contain set bits only for the samples that
are covered by the current fragment shader invocation, but the VGPR
initialization hardware loads the set of all bits that are covered at the
current pixel.

Fixes various tests in
dEQP-GLES31.functional.shaders.sample_variables.sample_mask_in.*

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>