Comment # 7 on bug 75127 from Alex Deucher

(In reply to comment #6)
> Compiled and installed Linux 3.12.12 kernel. No GPU hang problems occur for
> suspend and resume. Works fine (other than the fact that the laptop's
> display connected through LVDS is blank).
>

Does disabling dpm help?  Boot with radeon.dpm=0 on the kernel command line in
grub.  If not, can you bisect the kernel with git to find out what commit
caused the regression?