https://bugs.freedesktop.org/show_bug.cgi?id=77394
--- Comment #5 from Alex Deucher agd5f@yahoo.com --- It might be easier to just test Christian's fixes branch here: http://cgit.freedesktop.org/~deathsimple/linux/log/?h=drm-fixes-3.15-wip
Does disabling dpm help? Append radeon.dpm=0 to the kernel command line in grub. If that helps, the patches I mentioned may help.