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: NDavid Herrmann <dh.herrmann@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2059,16 +2059,10 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 			 * our own buffer, now let the real VM do its job and
 			 * go down in flames if truly OOM.
 			 */
-			gfp &= ~(__GFP_NORETRY | __GFP_NOWARN | __GFP_NO_KSWAPD);
-			gfp |= __GFP_IO | __GFP_WAIT;
-
 			i915_gem_shrink_all(dev_priv);
-			page = shmem_read_mapping_page_gfp(mapping, i, gfp);
+			page = shmem_read_mapping_page(mapping, i);
 			if (IS_ERR(page))
 				goto err_pages;
-
-			gfp |= __GFP_NORETRY | __GFP_NOWARN | __GFP_NO_KSWAPD;
-			gfp &= ~(__GFP_IO | __GFP_WAIT);
 		}
 #ifdef CONFIG_SWIOTLB
 		if (swiotlb_nr_tbl()) {