media: v4l2-ctrls: use media_request_(un)lock_for_access

When getting control values from a completed request, we have
to protect the request against being re-inited when it is
being accessed by calling media_request_(un)lock_for_access.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>