dma-direct: add missing set_memory_decrypted() for coherent mapping

When a coherent mapping is created in dma_direct_alloc_pages(), it needs
to be decrypted if the device requires unencrypted DMA before returning.

Fixes: 3acac065

 ("dma-mapping: merge the generic remapping helpers into dma-direct")
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>