In 3.16, GDM keeps a login screen running on vt1.
This login screen starts an Xwayland instance.
Since it's the first X server to start, it gets
the prized :0 display number.
This commit works around that problem, for now,
by having GDM's display number start at 1024.
https://bugzilla.gnome.org/show_bug.cgi?id=746295