gdm: Only start in runlevel 5

Runlevel 5 is traditionally graphical. If it starts in runlevel 2
or 3 systemd gets confused and adds it as a dependency of the
multi-user target, not just the graphical target.

Signed-off-by: Dan McGregor <dan.mcgregor@usask.ca>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>