media: i2c: gc2145: introduce enable/disable_streams ops

Introduce enable_streams and disable_streams pad ops and
replace s_stream with the v4l2_subdev_s_stream_helper
function.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>