iommu/dma: Refactor the page array remapping allocator

Move the call to dma_common_pages_remap into __iommu_dma_alloc and
rename it to iommu_dma_alloc_remap.  This creates a self-contained
helper for remapped pages allocation and mapping.

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