Actually the message is clear and after a look at the code I think I have a clue about what is going on.
My 2.4 kernel have been builded with the rivafb frame buffered console driver.
The register set for the video controler have already been mapped by this driver (BAR0 & 1: for control regs and FB regs).
These reg sets might also be reserved by nvidia_kernel driver.
Does this make sense ?