PCI: Add BAR index argument to pci_mmap_page_range()

In all cases we know which BAR it is.  Passing it in means that arch code
(or generic code; watch this space) won't have to go looking for it again.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>