drm: Go back to using device_unit(pciN) for pci busid.

This isn't arbitrary; it has to match existing logic in userland in
libpciaccess, and until we extend the pci(4) ioctl interface, and
find some way to handle userland compatibility, we can't change it.

drm: Use real pci segment number, not pciN device unit.

drm: Remove local diffs for drm_pci_set_busid.

Instead, move it to its own file that can be safely used without
bringing in a link/run-time dependency on pci.