The good news is: chip southbridge/amd/rs780 unconditionally pulls in the graphics driver which is still executed per PCI ID, and not per notation of static.c
So it's completely enough to drop the graphics device from Config.lb/devicetree.cb and say
chip southbridge/amd/rs780 device pci 0.0 on end # HT 0x9600 device pci 1.0 on end # Internal Graphics P2P bridge 0x9602
The only downside is that the subsystem IDs won't be set, because the device will not be "on mainboard" anymore.
Thanks, Myles