drm/i915: don't clflush gem objects in stolen memory

drm/i915: don't clflush gem objects in stolen memory

As explained by Chris Wilson gem objects in stolen memory are always
coherent with the GPU so we don't need to ever flush the CPU caches for
these.

This fixes a breakage - at least with the compact sg patches applied -
during the resume/restore gtt mappings path, when we tried to clflush an
FB object in stolen memory, but since stolen objects don't have backing
pages we passed an invalid page pointer to drm_clflush_page().

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>