We can move vfe_disable() into a common routine in the core VFE file
provided we make wm_stop() a VFE specific callback.

The callback is required to capture the case where VFE 17x currently isn't
VC enabled where as VFE 480 is.
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>