x86/PCI: MMCONFIG: trivial is_mmconf_reserved() interface simplification

Since pci_mmcfg_region contains the struct resource, no need to pass the
pci_mmcfg_region *and* the resource start/size.

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>