x86/PCI: merge pcibios_scan_root() and pci_scan_bus_on_node()

pcibios_scan_root() and pci_scan_bus_on_node() were almost identical,
so this patch merges them.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>