I don't think it's the kernel (3.12 or 3.13) , when I managed to make it work I
tried to boot under kernel 3.11 and it worked. 

Anyhow, today it doesn't work anymore, no matter what I do.
I didn't touch a thing, it was working fine for 2-3 days and now it's not.

boots fine with radeon.dpm=0