dma-mapping: merge direct and noncoherent ops

All the cache maintainance is already stubbed out when not enabled,
but merging the two allows us to nicely handle the case where
cache maintainance is required for some devices, but not others.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com> # MIPS parts

arch/mips/Kconfig

@@ -1121,7 +1121,6 @@ config DMA_NONCOHERENT
 	select NEED_DMA_MAP_STATE
 	select DMA_NONCOHERENT_MMAP
 	select DMA_NONCOHERENT_CACHE_SYNC
-	select DMA_NONCOHERENT_OPS
 
 config SYS_HAS_EARLY_PRINTK
 	bool