PCI: vmd: Create physical offset helper

Move the guest-passthrough physical offset discovery code to a new helper.
No functional changes.

Link: https://lore.kernel.org/r/20200728194945.14126-2-jonathan.derrick@intel.com


Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>