x86/PCI: Refine PCI support check in pcibios_init()

Also consider raw_pci_ext_ops when validating if a system has PCI support.
This leads to proper resource allocation via pcibios_resource_survey() in
the case where PCI config space is exclusively accessed through MMCONFIG.

Signed-off-by: Adrian-Ken Rueegsegger <ken@codelabs.ch>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>