A PCI function with its own _PXM still inherits a DMA tag carrying
the upstream bridge's proximity domain. Resolving an SR-IOV VF's
locality through its PF therefore does not affect the domain used for
DMA allocations.

Create and cache a private child tag when the function, or a VF's
owning PF, has an explicit _PXM. Parent it to the existing PCI or IOMMU
tag so its constraints remain intact, then apply the function's domain
without mutating a shared tag.

pci_get_dma_tag() already performs the IOMMU lookup, so remove the
duplicated lookup in the ACPI subclass while here.

MFC after:      2 weeks