https://bugs.freedesktop.org/show_bug.cgi?id=79806
--- Comment #6 from Chernovsky Oleg adonai@xaker.ru ---
It simply works without any trick. It also works with stock kernel 3.13. Are you sure that you have the same ATI card?
No, I have Radeon R7 260X, so it uses RadeonSI mesa driver. But I did some research and it appears Radeon DPM is now turned on by default (on latest kernel 3.15) and people complain abount non-working /proc variables everywhere. So sadly, currently there's no clue for me.