backend/drm: stop initializing renderer for parent backend

Unless we're dealing with a multi-GPU setup and the backend being
initialized is secondary, we don't need a renderer nor an allocator.
Stop initializing these.

include/backend/drm/renderer.h

@@ -13,7 +13,6 @@ struct wlr_buffer;
 
 struct wlr_drm_renderer {
 	struct wlr_drm_backend *backend;
-	struct gbm_device *gbm;
 
 	struct wlr_renderer *wlr_rend;
 	struct wlr_allocator *allocator;