i915: don't map imported dma-bufs for dmar.

The exporter should have given us pages in the correct place, avoid
the prepare object mapping phase on dmar systems.

This fixes an oops on a GM45/R600 machine, when running the intel/radeon
tests.
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -361,7 +361,8 @@ int i915_gem_gtt_prepare_object(struct drm_i915_gem_object *obj)
 	struct drm_device *dev = obj->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	if (dev_priv->mm.gtt->needs_dmar)
+	/* don't map imported dma buf objects */
+	if (dev_priv->mm.gtt->needs_dmar && !obj->sg_table)
 		return intel_gtt_map_memory(obj->pages,
 					    obj->base.size >> PAGE_SHIFT,
 					    &obj->sg_list,