Thanks for the patch. I still need radeon.bapm=0 to boot properly, but with
setting

pi->enable_nb_dpm = false;

the clock is getting higher and so I have the full performance. Finally I can
watch HD videos with VDPAU now :)

I run this tests using kernel 3.18-rc1. I do not have looked at the code
further, that is the consequence of this setting? Could there be any matching
BIOS setting? How can I further debug why my system is needing this change?