In 7f838b4dde, 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.