backend/drm: add special case for cursor in add_plane

The code calling add_plane now makes sure not to add a cursor plane to a
CRTC which already has one.

backend/drm/drm.c

@@ -92,10 +92,7 @@ static bool add_plane(struct wlr_drm_backend *drm,
 		struct wlr_drm_crtc *crtc, drmModePlane *drm_plane,
 		uint32_t type, union wlr_drm_plane_props *props) {
 	assert(!(type == DRM_PLANE_TYPE_PRIMARY && crtc->primary));
-
-	if (type == DRM_PLANE_TYPE_CURSOR && crtc->cursor) {
-		return true;
-	}
+	assert(!(type == DRM_PLANE_TYPE_CURSOR && crtc->cursor));
 
 	struct wlr_drm_plane *p = calloc(1, sizeof(*p));
 	if (!p) {