media: vim2m: add buf_out_validate callback

Validate the field for an output buffer. This ensures that the
field is validated when the buffer is queued to a request, and
not when the request itself is queued, which is too late.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>