Comment # 13 on bug 108521 from Christian König

This problem isn't related to the GPU in any way, that the amdgpu driver fails
to load is just another symptom.

The Thunderbolt bridge doesn't get enough resources assigned for its devices
even when the GPU isn't present at all for some reason.

That could be a problem with the BIOS, with the Linux Thunderbolt or the
resource allocation in the PCI subsystem.

Please provide the output of "sudo cat /proc/iomem" and "lspci -t -nn -v"
together with an up to date dmesg.