x86/PCI: make pci=lastbus=255 work when acpi is on

Impact: scan more peer root buses even acpi is used

Move pci_bios_fixup_peer_bridges out of pci_legacy_init and into
pci_subsys_init.  This allows pci_bios_fixup_peer_bridges to be called
even pci_apci_init is driving PCI initialization.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>