media: vb2: Fix buf_out_validate documentation

The .buf_out_validate callback is mandatory for OUTPUT
queues. Mark it as such in the callback's doc.

Fixes: 28d77c21 ("media: vb2: add buf_out_validate callback")

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