media: imx319: Enable runtime PM before registering async sub-device

As the sensor may be accessible right after its async sub-device is
registered, enable runtime PM before doing so.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>