I followed the solution to this question: pci-stub not claiming radeon card, only the audio device, radeon driver claiming graphics instead
Now PCI_stub is claiming the devices (video and audio of my HD7970) properly, but as I have two AMD GPUs, I need the Radeon driver loaded, but this is not happening.
I can not find a solution as google is mostly pointing at the thread mentioned above and other threads are pretty old or for other linux distros.
Any idea what to do? I will happily provide any additional information needed.