https://bugs.freedesktop.org/show_bug.cgi?id=66942
--- Comment #11 from Alex Deucher agd5f@yahoo.com --- Does it boot correctly if you build radeon as a module, then disable radeon (add radeon.modeset=0 to your kernel command line in grub) and boot to a non-X runlevel, then load radeon manually?
modprobe -r radeon modprobe radeon modeset=1