From what I see is that you use the wrong format, it's RGBFB_something not RGBFF_something. Additionally, the Radeon driver only supports RGBFB_YUV422CGX.
IIRC it must be MEMF_SHARED | MEMF_HWALIGNED but I could be wrong because I never had the honour to write a DMAable driver. For GetPhysicalAddress() and friends you need to switch to Supervisor() mode, first.