PCI: Remove unused pci_get_dma_source()

pci_get_dma_source() is unused, so remove it.  We now have
dma_alias_devfn() to describe this.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>