[media] vivid-vid-out: use memdup_user()

Instead of allocating and coping from __user, do it using
one atomic call. That makes the code simpler.

Found by coccinelle.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>