[media] saa7134: add DMABUF support

Since saa7134 is now using vb2, there is no reason why we can't support
dmabuf for this driver.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>