systemd: make sure to only start plymouth is started
    
    This commit makes sure gdm doesn't start in parallel with plymouth,
    which could lead to plymouth activating after X is started.
    
    Furthermore, GDM is replacing getty@tty1.service and
    plymouth-quit.service, so it needs to have the same dependencies
    as them.
    
    https://bugs.freedesktop.org/show_bug.cgi?id=89475