drm/i915: check that the i965g/gm 4G limit is really obeyed

In truly crazy circumstances shmem might give us the wrong type of
page. So be a bit paranoid and double check this.

Reviewer: Damien Lespiau <damien.lespiau@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
References: http://lkml.org/lkml/2011/7/11/238
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1902,6 +1902,9 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 			sg->length += PAGE_SIZE;
 		}
 		last_pfn = page_to_pfn(page);
+
+		/* Check that the i965g/gm workaround works. */
+		WARN_ON((gfp & __GFP_DMA32) && (last_pfn >= 0x00100000UL));
 	}
 #ifdef CONFIG_SWIOTLB
 	if (!swiotlb_nr_tbl())