video: mmp: delete a stray mutex_unlock()

We aren't holding the disp_lock so we shouldn't release it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>