dma-contiguous: cleanup dma_alloc_contiguous

Split out a cma_alloc_aligned helper to deal with the "interesting"
calling conventions for cma_alloc, which then allows to the main
function to be written straight forward.  This also takes advantage
of the fact that NULL dev arguments have been gone from the DMA API
for a while.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nicolin Chen <nicoleotsuka@gmail.com>
Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>