backend/drm: drop wlr_drm_layer.pending_{width,height}

No need to store this info in struct wlr_drm_layer. We can just
extract the size when we need it.

include/backend/drm/drm.h

@@ -50,8 +50,6 @@ struct wlr_drm_layer {
 	/* Buffer currently displayed on screen */
 	struct wlr_drm_fb *current_fb;
 
-	int pending_width, pending_height;
-
 	// One entry per wlr_drm_backend.planes
 	bool *candidate_planes;
 };