anv: Implement the Skylake stencil PMA optimization

anv: Implement the Skylake stencil PMA optimization

Unfortunately, this doesn't substantially improve the performance of any
known apps.  With Dota 2 on my Sky Lake gt4, it seems help by somewhere
between 0% and 1% but there's enough noise that it's hard to get a clear
picture.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>