The DMA API does its own zone decisions based on the coherent_dma_mask.

[ Note: as the driver doesn't set the DMA coherent mask, we can assume
  the default 32bit DMA, hence it should be safe to drop the flag here
  -- tiwai ]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>