[media] media: vb2: Share code between vb2_prepare_buf and vb2_qbuf

The two operations are very similar, refactor most of the code in a
helper function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>