[media] v4l: vsp1: Fix mutex double lock at streamon time

A mutex_lock() was left when the driver was converted to use the vb2
ioctl helpers, resulting in a deadlock at streamon time. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>