That the messages in comment 1 are merely kernel warnings, no real crash log, and it doesn't mean directly a cause of the crash, per se. If the upstream kernel shows the same behavior but no log, it might be a different reason that triggers the reboot. At best, you can try to boot with drm.debug=0x1e boot option to get more verbose logs from DRM. It might show something useful. Please attach the full kernel log until it faces the reboot.