Well, I managed by using pmap functions to get the legacy Video BIOS 
mapped into the kernel memory space and then into the right radeonfb 
buffer. At least, that works fine. If you are interested in it, I can 
mail up the diff file for it.

Anyhow, I stumbled upon another problem. My dmesg looks now like this:

radeonfb0: no PCI BIOS, trying legacy ISA address...
radeonfb0: found 128 KB Legacy BIOS
DFP table revision 2

And the radeonfb code deals only with DFP Tables revision 3. Any hints 
to help me go further?

Vincent