Comment # 25 on bug 958880 from Dr. Werner Fink

(In reply to Marcus Meissner from comment #20)

Hmmm ... after

  systemctl enable display-manager.service

there should be a link xdm.service below /etc/systemd/system which points to
display-manager.service below /usr/lib/systemd/system

The only questions: Does xdm package use

 %pre
 %service_add_pre display-manager.service

 %post
 %service_add_post display-manager.service

 %preun
 %service_del_preun display-manager.service

 %postun
 %service_del_postun display-manager.service

and is display-manager.service enabled below

 /usr/lib/systemd/system-preset/