anv: Stop doing too much per-sample shading

anv: Stop doing too much per-sample shading

We were setting anv_pipeline::sample_shading_enable based on
sampleShadingEnable without looking at minSampleShading.  We would then
pass this value into nir_lower_wpos_center which would add sample_pos to
frag_coord.  Then the back-end compiler picks up on the existence of
sample_pos and forces persample dispatch.  This leads to doing
per-sample dispatch whenever sampleShadingEnable = VK_TRUE regardless of
the value of minSampleShading.  This is almost certainly costing us
perf somewhere.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14022>