backend/drm: ensure disconnected outputs are disabled after VT switch

The following situation can be dangerous:

- Output DP-1 is plugged in, compositor enables it.
- User VT switches away.
- User unplugs DP-1.
- User VT switches back.
- scan_drm_connectors() figures out the output is now disconnected,
  uninitializes the struct wlr_output.
- The loop restoring previous output state in handle_session_active()
  accesses the struct wlr_output to figure out what to restore.

By chance, we zero out the struct wlr_output after uninitializing it,
so enabled and current_mode will always be zero. But let's make sure
we handle this case explicitly, to remind future readers that it exists
and make the code less fragile.

backend/drm/backend.c

@@ -107,7 +107,8 @@ static void handle_session_active(struct wl_listener *listener, void *data) {
 		wl_list_for_each(conn, &drm->outputs, link) {
 			struct wlr_output_mode *mode = NULL;
 			uint32_t committed = WLR_OUTPUT_STATE_ENABLED;
-			if (conn->output.enabled && conn->output.current_mode != NULL) {
+			if (conn->status != DRM_MODE_DISCONNECTED && conn->output.enabled
+					&& conn->output.current_mode != NULL) {
 				committed |= WLR_OUTPUT_STATE_MODE;
 				mode = conn->output.current_mode;
 			}