Discrete graphics are not dependent on the processor instruction set, they just need to interface with the PCI bus. All AMD has to do is deliver their cards, the drivers are FLOSS already.
I assume the drivers might have a lot of specific x86 optimizations, so it's easy (but not too easy)
I'm not sure how's the status of PCI-X on Arm machines at the moment as well. Do PCs still have the North and South bridge to interface to multiple devices?
