dma-contiguous: fix the Kconfig entry for CONFIG_DMA_NUMA_CMA

It makes no sense to expose CONFIG_DMA_NUMA_CMA if CONFIG_NUMA is not
enabled, and random config options shouldn't be default unless there
is a good reason.  Replace the default NUMA with a depends on to fix both
issues.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <roin.murphy@arm.com>

kernel/dma/Kconfig

@@ -160,7 +160,7 @@ if  DMA_CMA
 
 config DMA_NUMA_CMA
 	bool "Enable separate DMA Contiguous Memory Area for NUMA Node"
-	default NUMA
+	depends on NUMA
 	help
 	  Enable this option to get numa CMA areas so that NUMA devices
 	  can get local memory by DMA coherent APIs.