media: ov5647: Rework s_stream() operation

Rework the s_stream() operation to turn the sensor on and
off at stream enable/disable time using the pm_runtime infrastructure.

Protect the stream on/off from being called multiple times in
sequence with a 'streaming' flag.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>