dma-direct: don't retry allocation for no-op GFP_DMA

When an allocation with lower dma_coherent mask fails, dma_direct_alloc()
retries the allocation with GFP_DMA.  But, this is useless for
architectures that hav no ZONE_DMA.

Fix it by adding the check of CONFIG_ZONE_DMA before retrying the
allocation.

Fixes: 95f18391 ("dma-direct: retry allocations using GFP_DMA for small masks")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>