drm/mm: WARN for unclean mm takedown

The usual drm driver has tons of different drm_mm memory managers so the drm
error message in dmesg is pretty useless. WARN instead so that we have the full
backtrace.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_mm.c

@@ -697,8 +697,8 @@ void drm_mm_takedown(struct drm_mm * mm)
 {
 	struct drm_mm_node *entry, *next;
 
-	if (!list_empty(&mm->head_node.node_list)) {
-		DRM_ERROR("Memory manager not clean. Delaying takedown\n");
+	if (WARN(!list_empty(&mm->head_node.node_list),
+		 "Memory manager not clean. Delaying takedown\n")) {
 		return;
 	}