uterm: add PCI primary GPU detection

uterm: add PCI primary GPU detection

We used to simply probe every GPU that is reported by the kernel. However,
if a system has multiple GPUs that share display controllers, we cannot
use both simultaneously. Unfortunately, the kernel currently does not
notify us about this. Hence, we use some heuristics to determine which GPU
is the boot-gpu/primary-gpu.

This only adds the detection logic, it does not modify any code to use
this detection at all.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>