В задании upstart lightdm (/etc/init/lightdm.conf) добавьте смонтированное событие, чтобы оно выглядело примерно так:
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)