display-manager: Require systemd-udev-settle.service

It was already ordered after systemd-udev-settle.service, but that
doesn't do anything if no other units require
systemd-udev-settle.service. This was causing random failures during X
server startup, e.g.

machine# [   12.691372] display-manager[607]: (EE) open /dev/dri/card0: No such file or directory

http://hydra.nixos.org/build/41062823