media: i2c: imx296: Replace streaming flag with runtime PM check

The streaming flag in the driver private structure is used for the sole
purpose of gating register writes when setting a V4L2 control. This is
better handled by checking if the sensor is powered up using the runtime
PM API. Do so and drop the streaming flag.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tommaso Merciai <tomm.merciai@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>