https://bugs.freedesktop.org/show_bug.cgi?id=110509
--- Comment #14 from James.Dutton@gmail.com --- I stop gdm and kill any remaining X processes. When I start gdm and login, it works, and displays the desktop.
Previously, I was leaving on of the X processes running.
So, I think this (drm-next-5.2-wip) has fixed this bug.