dma-direct: refactor the !coherent checks in dma_direct_alloc

Add a big central !dev_is_dma_coherent(dev) block to deal with as much
as of the uncached allocation schemes and document the schemes a bit
better.

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