wayland: store display FD in wayland struct

This way we:

* Don't have to call wl_display_get_fd() all the time
* No longer call fdm_del_no_close() even though the FD hasn't been
  added to the FDM.

wayland.h

@@ -198,6 +198,9 @@ struct terminal;
 struct wayland {
     const struct config *conf;
     struct fdm *fdm;
+
+    int fd;
+
     struct wl_display *display;
     struct wl_registry *registry;
     struct wl_compositor *compositor;