PCI/P2PDMA: Factor out __upstream_bridge_distance()

PCI/P2PDMA: Factor out __upstream_bridge_distance()

This is a prep patch to create a second level helper. There are no
functional changes.

The root complex whitelist code will be moved into this function in a
subsequent patch.

Link: https://lore.kernel.org/r/20190730163545.4915-5-logang@deltatee.com
Link: https://lore.kernel.org/r/20190812173048.9186-5-logang@deltatee.com
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>