https://bugs.freedesktop.org/show_bug.cgi?id=91893
--- Comment #5 from macro@hotmail.com --- Kernel arguments:
radeon.no_wb=1 radeon.dynclks=0 radeon.agpmode=-1 radeon.gartsize=32 radeon.msi=0 radeon.dpm=0 radeon.aspm=0 radeon.runpm=0 radeon.hard_reset=1 radeon.use_pflipirq=0 radeon.bapm=0
One or more (not sure exactly which) are required in order for X to start without hanging the machine.