media: i2c: ov5645: Drop `power_lock` mutex

Remove the `power_lock` mutex used during control applications, as it is
only utilized in the .s_ctrl() function. Since the control framework
already serializes calls to this function, the mutex is unnecessary.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>