https://bugs.freedesktop.org/show_bug.cgi?id=65068
--- Comment #5 from Alex Deucher agd5f@yahoo.com --- (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.