media: vimc: Initialize subdev active state

Finalize subdev initialization for all subdevs that provide a
.init_state() operation. This creates an active state for all those
subdevs, which subsequent patches will use to simplify the
implementation of individual vimc entities.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>