https://bugs.freedesktop.org/show_bug.cgi?id=75127
--- Comment #15 from Alex Deucher agd5f@yahoo.com --- Please try these patches: http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.14&id=9ba... http://cgit.freedesktop.org/~agd5f/linux/commit/?h=drm-fixes-3.14&id=784...
You can also force runpm off by booting with radeon.runpm=0 on the kernel command line in grub.