iommu/dma: Refactor iommu_dma_alloc

Shuffle around the self-contained atomic and non-contiguous cases to
return early and get out of the way of the CMA case that we're about to
work on next.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[hch: slight changes to the code flow]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>