[media] uvcvideo: Implement DMABUF exporter role

Now that videobuf2-vmalloc supports exporting buffers, add support for
the DMABUF exporter role by plugging in the videobuf2 ioctl helper.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>