iommu/dma: Move __iommu_dma_map

Moving this function up to its unmap counterpart helps to keep related
code together for the following changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>