dma-mapping: make dma_atomic_pool_init self-contained

The memory allocated for the atomic pool needs to have the same
mapping attributes that we use for remapping, so use
pgprot_dmacoherent instead of open coding it.  Also deduct a
suitable zone to allocate the memory from based on the presence
of the DMA zones.

Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/dma-mapping.h

@@ -624,7 +624,6 @@ void *dma_common_pages_remap(struct page **pages, size_t size,
 			const void *caller);
 void dma_common_free_remap(void *cpu_addr, size_t size, unsigned long vm_flags);
 
-int __init dma_atomic_pool_init(gfp_t gfp, pgprot_t prot);
 bool dma_in_atomic_pool(void *start, size_t size);
 void *dma_alloc_from_pool(size_t size, struct page **ret_page, gfp_t flags);
 bool dma_free_from_pool(void *start, size_t size);