anv: Add stencil texturing support for gen7

anv: Add stencil texturing support for gen7

Intel hardware didn't get support for sampling from W-tiled (required
for stencil) images until Broadwell so we can't directly sample from
stencil.  Instead, if we want to support stencil texturing on gen7
hardware, we have to keep a texture-capable shadow copy around and use
BLORP to update when stencil changes.  The one thing this commit does
not implement is self-dependencies with stencil input attachments.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99493
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>