dma-mapping: DMA_COHERENT_POOL should select GENERIC_ALLOCATOR

The dma coherent pool code needs genalloc.  Move the select over
from DMA_REMAP, which doesn't actually need it.

Fixes: dbed452a

 ("dma-pool: decouple DMA_REMAP from DMA_COHERENT_POOL")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David Rientjes <rientjes@google.com>