Instead of sprinkling the device's DMA tag, always return a new DMA tag

Instead of sprinkling the device's DMA tag, always return a new DMA tag
which is based on the IOMMU's.  If you think about it, using the IOMMU's
DMA tag makes more sense because it is the IOMMU that does the actual DMA.
Noticed while debugging, since the SMMU's map function was called twice:
once for the PCI device, and once for its ppb(4).  As the transaction has
the PCI device's Stream ID, not the ppb(4)'s, this would be useless work.

Suggested by kettenis@