x86/mm: unencrypted non-blocking DMA allocations use coherent pools

When CONFIG_AMD_MEM_ENCRYPT is enabled and a device requires unencrypted
DMA, all non-blocking allocations must originate from the atomic DMA
coherent pools.

Select CONFIG_DMA_COHERENT_POOL for CONFIG_AMD_MEM_ENCRYPT.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>