PCI: Remove DPC tristate module option

Change the Downstream Port Containment config type from tristate to bool.

The driver doesn't automatically load based on any rules, so it needs to be
built-in in order to bind to devices it needs to drive.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>