iommu/dma: Move domain lookup into __iommu_dma_{map,unmap}

Most of the callers don't care, and the couple that do already have the
domain to hand for other reasons are in slow paths where the (trivial)
overhead of a repeated lookup will be utterly immaterial.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[hch: dropped the hunk touching iommu_dma_get_msi_page to avoid a
 conflict with another series]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>