[media] vb2-dma-sg: add support for dmabuf exports

Add DMABUF export support to vb2-dma-sg.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Acked-by: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>