Comment # 5 on bug 65068 from Alex Deucher

(In reply to comment #4)
> I'm not sure if this has anything to do with this bug, but the PCI config
> space is all messed up when running on the integrated GPU:

When you disable the GPU the hardware is physically powered off so you are
accessing non-existent registers.  You need to power up the GPU using
vgaswitcheroo before loading/resuming the driver or access the config space
with lspci.