No trabalho inicial do lightdm (/etc/init/lightdm.conf), adicione um evento montado para que fique parecido com isto:
start on ((filesystem
and runlevel [!06]
and started dbus
and (drm-device-added card0 PRIMARY_DEVICE_FOR_DISPLAY=1
or stopped udev-fallback-graphics))
and mounted MOUNTPOINT=/home
or runlevel PREVLEVEL=S)