x86/PCI: simplify pci_mmcfg_late_insert_resources()

Reduce redundant code to simplify pci_mmcfg_late_insert_resources().

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jiang Liu <liuj97@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>