PCI: provide stub pci_domain_nr function for !CONFIG_PCI configs

Allows the new PCI domain aware DRM code to compile on m68k.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>