x11/{lightdm,sddm}: add CONFLICTS

Both lightdm and sddm install the DBus file for DisplayManager,
etc/dbus-1/system.d/org.freedesktop.DisplayManager.conf
It doesn't really make sense to install multiple DMs on one
machine, but for completeness in the ports framework, mark
them as conflicting.

PR: 256688 256687
Reported by: Graham Perrin