iommu/dma: Refactor iommu_dma_get_sgtable

Inline __iommu_dma_get_sgtable_page into the main function, and use the
fact that __iommu_dma_get_pages return NULL for remapped contigous
allocations to simplify the code flow a bit.

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