drm/armada: add work cancel callback

Add a work cancel callback, so that work items can add functionality to
clean themselves up when they are cancelled.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

drivers/gpu/drm/armada/armada_crtc.h

@@ -37,6 +37,7 @@ struct armada_variant;
 
 struct armada_plane_work {
 	void (*fn)(struct armada_crtc *, struct armada_plane_work *);
+	void (*cancel)(struct armada_crtc *, struct armada_plane_work *);
 	struct drm_plane *plane;
 };