services feature works like this:

  1. enable whatever services are in DEFAULT_SERVICES_ENABLE
  2. disable whatever is in DEFAULT_SERVICES_DISABLE
  3. enable services in SERVICES_ENABLE
  4. finally, disable those in SERVICES_DISABLE

So one should not disable display-manager service!

Disabling prefdm is not needed as it doesn't start
when display-manager.service symlink exists.