i965: Fix gl_SampleMaskIn[] in per-sample shading mode.

i965: Fix gl_SampleMaskIn[] in per-sample shading mode.

The coverage mask is not sufficient - in per-sample mode, we also need
to AND with a mask representing the samples being processed by the
current fragment shader invocation.

Fixes 18 dEQP-GLES31.functional.shaders.sample_variables tests:

sample_mask_in.bit_count_per_sample.multisample_{rbo,texture}_{1,2,4,8}
sample_mask_in.bit_count_per_two_samples.multisample_{rbo,texture}_{4,8}
sample_mask_in.bits_unique_per_sample.multisample_{rbo,texture}_{1,2,4,8}
sample_mask_in.bits_unique_per_two_samples.multisample_{rbo,texture}_{4,8}

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>