https://bugzilla.kernel.org/show_bug.cgi?id=90851
--- Comment #4 from Michel Dänzer michel@daenzer.net --- (In reply to Christoph Haag from comment #3)
radeon: mmap failed, errno: 12
That's ENOMEM, so it looks like the kernel runs out of memory. Maybe a leak somewhere.
(In reply to Christoph Haag from comment #2)
But I am relatively sure that it did not happen with 3.18.
Can you bisect?