i965/fs: Use sample interpolation for interpolateAtCentroid in persample mode

i965/fs: Use sample interpolation for interpolateAtCentroid in persample mode

From the ARB_gpu_shader5 spec:

   The built-in functions interpolateAtCentroid() and interpolateAtSample()
   will sample variables as though they were declared with the "centroid"
   or "sample" qualifiers, respectively.

When running with persample dispatch forced by the API, we interpolate
anything that isn't flat as if it's qualified by "sample".  In order to
keep interpolateAtCentroid() consistent with the "centroid" qualifier, we
need to make interpolateAtCentroid() do sample interpolation instead.
Nothing in the GLSL spec guarantees that the result of
interpolateAtCentroid is uniform across samples in any way, so this is a
perfectly fine thing to do.

Fixes 8 of the new dEQP-VK.pipeline.multisample_interpolation.* Vulkan CTS
tests that specifically validate consistency between the "sample" qualifier
and interpolateAtSample()

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>