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>