anv: Enable per sample shading on gen8+

Vulkan CTS test results on gen9:
./deqp-vk --deqp-case=dEQP-VK.pipeline.multisample.min_sample_shading*
Test run totals:
  Passed:        60/90 (66.7%)
  Failed:        0/90 (0.0%)
  Not supported: 30/90 (33.3%)
  Warnings:      0/90 (0.0%)

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
2 files changed