drm/i915: use shmem helpers if possible

Instead of shuffling gfp-masks all the time, use the
shmem_read_mapping_page() helper. Note that __GFP_IO and __GFP_WAIT are
set in mapping_gfp_mask() for i915, so the behavior is still the same.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>