dma-direct: factor the swiotlb code out of __dma_direct_alloc_pages

Add a new helper to deal with the swiotlb case.  This keeps the code
nicely boundled and removes the not required call to
dma_direct_optimal_gfp_mask for the swiotlb case.

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