x86/PCI: Clean up and mark early_root_info_init() as deprecated

early_root_info_init() is now deprecated in favor of info in ACPI.  Add a
note to that effect.  Also, clean up the code a bit.

There is no functional change.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>