output/drm: don't use OUT_FENCE_PTR

The returned fence is not required to be signalled at the earliest
possible time. It is not intended to replace the drm flip event, and is
expected to be signalled only much later

(cherry picked from commit 8d454e1e34)

backend/drm/drm.c

@@ -668,7 +668,6 @@ static void drm_connector_state_init(struct wlr_drm_connector_state *state,
 		.base = base,
 		.active = output_pending_enabled(&conn->output, base),
 		.primary_in_fence_fd = -1,
-		.out_fence_fd = -1,
 	};
 
 	struct wlr_output_mode *mode = conn->output.current_mode;