drm/i915/gtt: Unmap the PCI pages after unbinding them from the GTT

Dave Airlie spotted that his ILK laptop with DMAR enabled was generating
the occasional DMAR warning.

"The ordering in the previous code was to rewrite the GTT table before
unmapping the pages and that makes sense to me."

This is his stable patch ported to d-i-n.
Reported-by: NDave Airlie <airlied@redhat.com>
Original-patch-by: NDave Airlie <airlied@redhat.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -85,15 +85,11 @@ int i915_gem_gtt_bind_object(struct drm_i915_gem_object *obj)
 
 void i915_gem_gtt_unbind_object(struct drm_i915_gem_object *obj)
 {
-	struct drm_device *dev = obj->base.dev;
-	struct drm_i915_private *dev_priv = dev->dev_private;
+	intel_gtt_clear_range(obj->gtt_space->start >> PAGE_SHIFT,
+			      obj->base.size >> PAGE_SHIFT);
 
-	if (dev_priv->mm.gtt->needs_dmar) {
+	if (obj->sg_list) {
 		intel_gtt_unmap_memory(obj->sg_list, obj->num_sg);
 		obj->sg_list = NULL;
-		obj->num_sg = 0;
 	}
-
-	intel_gtt_clear_range(obj->gtt_space->start >> PAGE_SHIFT,
-			      obj->base.size >> PAGE_SHIFT);
 }