compositor-drm: handle null cursor_plane

Was crashing when I tried to take a screenshot.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-drm.c

@@ -1828,9 +1828,11 @@ drm_output_apply_state_legacy(struct drm_output_state *state)
 	 */
 	if (output->base.disable_planes) {
 		output->cursor_view = NULL;
+		if (output->cursor_plane) {
 			output->cursor_plane->base.x = INT32_MIN;
 			output->cursor_plane->base.y = INT32_MIN;
 		}
+	}
 
 	if (state->dpms != WESTON_DPMS_ON) {
 		wl_list_for_each(ps, &state->plane_list, link) {