backend/drm: Close non-master drm fd on failure

If we are not able to prepare the fd for non-master usage, close the fd
before returning an error.

(cherry picked from commit 1efb216c6d)

backend/drm/drm.c

@@ -2085,6 +2085,7 @@ int wlr_drm_backend_get_non_master_fd(struct wlr_backend *backend) {
 
 	if (drmIsMaster(fd) && drmDropMaster(fd) < 0) {
 		wlr_log_errno(WLR_ERROR, "Failed to drop master");
+		close(fd);
 		return -1;
 	}