I just ran headlong into this and realized that I had at some point replaced my go and syslinux.cfg without adding back the i915 modifications that were required.
This thread was more useful to me at this point in time than the reddit thread (looks like some stuff was deleted from it): https://forums.unraid.net/topic/62525-solved-igpu-transcoding-i915-driver-present-but-devdri-not-available/?do=findComment&comment=703961