Probably a bit of everything you suggested. Modesetting changed due to Eyefinity and needs to be rewritten, they're currently working on that (but stumbled over some bugs with DVI outputs).
DRM usually needs to be updated to at least recognize the new GPU, probably needs to add additional informations like changed register locations (or semantics), updated AtomBIOS specs etc.
radeon or radeonhd need to be made aware of the new GPUs as well, not sure about other changes required.
A new shader compiler needs to be written (or the previous one extended) to fit the reworked shader hardware.
Probably a lot of other changes I'm not aware of.
Gallium does not (yet) fit in here, as it isn't widely used nor actually needed yet. Eventually, a G3D driver will need to be written as well, though.