Ignore the cache bits in PPAT and just set the snoop bit where
appropriate. BDW WB is mapped to snooped access, while all other
modes are mapped to non-snooped access.

The hardware supposedly ignores everything except the snoop bit
in the PPAT entries.

Additionally the hardware actually enforces snooping for all
page table accesses, and thus the snoop bit is ignored for PDEs.

v2: Rebased on top of the bdw resume fix to reload the ppat entries.

v3: Rebase on top of the i915_gem_gtt.h header extraction.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (v1)
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Rafael Barbalho <rafael.barbalho@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>