backend/drm: log when creating multi-GPU renderer

Creating a renderer results in lots of logs. Make it clear that
the logs belong to a multi-GPU renderer (as opposed to a primary
renderer created by the compositor).

backend/drm/renderer.c

@@ -14,6 +14,7 @@
 
 bool init_drm_renderer(struct wlr_drm_backend *drm,
 		struct wlr_drm_renderer *renderer) {
+	wlr_log(WLR_DEBUG, "Creating multi-GPU renderer");
 	renderer->wlr_rend = renderer_autocreate_with_drm_fd(drm->fd);
 	if (!renderer->wlr_rend) {
 		return false;