drm/i915: Ensure that the mode change flushing is currently uninterruptible

Introduced by 48b956c5, I had thought I had already fixed this. Oh well.
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -2858,10 +2858,17 @@ i915_gem_object_set_to_display_plane(struct drm_gem_object *obj,
 	if (obj_priv->gtt_space == NULL)
 		return -EINVAL;
 
-	ret = i915_gem_object_flush_gpu_write_domain(obj, pipelined);
+	ret = i915_gem_object_flush_gpu_write_domain(obj, true);
 	if (ret)
 		return ret;
 
+	/* Currently, we are always called from an non-interruptible context. */
+	if (!pipelined) {
+		ret = i915_gem_object_wait_rendering(obj, false);
+		if (ret)
+			return ret;
+	}
+
 	i915_gem_object_flush_cpu_write_domain(obj);
 
 	old_read_domains = obj->read_domains;