Multiple DRI card detection in compositor systemd units

[Simon Ser]

Maybe try creating multiple physical seats with logind, and start each
compositor on its own seat? A physical seat is a collection of devices like
DRM nodes and evdev device files.

Also udev creates files in /dev/dri/by-path/, these should be stable across
reboots. `udevadm settle` before a compositor start-up can wait for udev to
finish its job.

Out of curiosity, can you explain your use-case? Why do you need to start
multiple compositors, each on its own GPU?