I just discovered today that this bug is related to (and is perhaps the fundamental cause of) other issues I've been seeing.
Specifically, systemd-logind not being able to recognize my NIS user account resulted in my not being able to start X (via startx) without hacking the Xwrapper.config to revert to running X as root. This was reported under Debian Bug#833182 because at the time, one of my symptoms (the "drmSetMaster(): Permission denied" error) matched the subject of that bug, and I am also on Intel graphics. So, it seemed like the same bug. The other symptoms, which I only noticed more recently, were the complete lack of all XDG_* environment variables and the lack of a /run/user/MYUID directory, all of which were supposed to be present in my login sessions. After installing nscd and rebooting (it didn't work until reboot), all of these symptoms have been cleared up. I was able to remove the needs_root_rights=yes line in the /etc/X11/Xwrapper.config file and start X without root again. I don't know if there's a less drastic measure than rebooting that can make nscd "kick in".
