gdm: do not rely on dev-tty7.device

Error message was:

[ TIME ] Timed out waiting for device dev-tty7.device.
[DEPEND] Dependency failed for Gnome Display Manager.

run tested on overo/xfce

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>