drm/sti: fix warning in build.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/sti/sti_compositor.c b/drivers/gpu/drm/sti/sti_compositor.c
index 770a725..390d93e 100644
--- a/drivers/gpu/drm/sti/sti_compositor.c
+++ b/drivers/gpu/drm/sti/sti_compositor.c
@@ -119,6 +119,8 @@
 						(1 << crtc) - 1, plane_type);
 				plane++;
 				break;
+			case STI_BCK:
+				break;
 			}
 
 			/* The first planes are reserved for primary planes*/