x86/PCI: Use x86_pci_root_bus_node() instead of get_mp_bus_to_node()

This replaces all uses of get_mp_bus_to_node() with x86_pci_root_bus_node().

I think these uses are all on root buses, except possibly for blind
probing, where NUMA node information is unimportant.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>