iommu/dma: Merge the CMA and alloc_pages allocation paths

Instead of having a separate code path for the non-blocking alloc_pages
and CMA allocations paths merge them into one.  There is a slight
behavior change here in that we try the page allocator if CMA fails.
This matches what dma-direct and other iommu drivers do and will be
needed to use the dma-iommu code on architectures without DMA remapping
later on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>