anv: Flatten descriptor bindings in anv_nir_apply_pipeline_layout

anv: Flatten descriptor bindings in anv_nir_apply_pipeline_layout

This lets us stop tracking the pipeline layout.  It also means less
indirection on a very hot path.  As an extra bonus, we can make some of
our data structures smaller.  No measurable CPU overhead improvement.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>