nixos/gdm: fix startup

In https://github.com/NixOS/nixpkgs/commit/7f838b4ddeadffbbe5863a6ca27ad776064a2f4a, we dropped systemd-udev-settle.service from display-manager.service's wants.
Unfortunately, we are doing something wrong since without it both Xorg and Wayland fail to start:

	Failed to open gpu '/dev/dri/card0': GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: Operation not permitted

Until we sort this out, let's add systemd-udev-settle.service to GDM to unblock the channels.