It would appear to my untrained eye, that this commit on May 15, 2018 by Ben Hutchings is responsible for this issue:
https://salsa.debian.org/kernel-team/linux/commit/e7bdd24f189b1b8775665d56a1230f7b12915ffe This appears to be debian specific and not an upstream issue as the upstream code doesn't contain any such checks: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/gpu/drm/radeon/radeon_drv.c?h=v4.19.59#n340 It would appear this is an incorrect assumption to make, that all cards newer than the R600 require firmware. I can't make out what this code is doing without digging way deeper but I think this is what is causing this issue. There would need to be an exception here or the patch would need to be rolled back or came at from a different angle entirely.
