videobuf2-dma-contig: Only support if HAVE_GENERIC_DMA_COHERENT

As reported by Stephen:

> After merging the v4l-dvb tree, today's linux-next build (x86_64
> allmodconfig) failed like this:
>
> ERROR: "dma_common_get_sgtable" [drivers/media/v4l2-core/videobuf2-dma-contig.ko] undefined!
>
> Caused by commit 9ef2cbeb ("[media] v4l: vb2-dma-contig: add support
> for DMABUF exporting").  dma_common_get_sgtable() is called from the
> inline dma_get_sgtable_attrs() and dma_get_sgtable_attrs() is "called
> from" dma_get_sgtable().  dma_common_get_sgtable() is only defined if
> ARCH_HAS_DMA_DECLARE_COHERENT_MEMORY is set which depends on
> CONFIG_HAVE_GENERIC_DMA_COHERENT being set which it isn't for this build.

The simplest fix is to just not support DMABUF with vb2-contig, on archs
where this symbol doesn't exist. Some latter patch can rewrite it to
support this outside -arm.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>