[media] vb2: add dma_dir to the alloc memop

This is needed for the next patch where the dma-sg alloc memop needs
to know the dma_dir.

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