media: ov772x: handle nested s_power() calls

Depending on the v4l2 driver, calling s_power() could be nested.  So the
actual transitions between power saving mode and normal operation mode
should only happen at the first power on and the last power off.

This adds an s_power() nesting counter and updates the power state if the
counter is modified from 0 to != 0 or from != 0 to 0.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>