media: pxa_camera: avoid duplicate s_power calls

The open() operation for the pxa_camera driver always calls s_power()
operation to put its subdevice sensor in normal operation mode, and the
release() operation always call s_power() operation to put the subdevice
in power saving mode.

This requires the subdevice sensor driver to keep track of its power
state in order to avoid putting the subdevice in power saving mode while
the device is still opened by some users.

Many subdevice drivers handle it by the boilerplate code that increments
and decrements an internal counter in s_power() like below:

	/*
	 * If the power count is modified from 0 to != 0 or from != 0 to 0,
	 * update the power state.
	 */
	if (sensor->power_count == !on) {
		ret = ov5640_set_power(sensor, !!on);
		if (ret)
			goto out;
	}

	/* Update the power count. */
	sensor->power_count += on ? 1 : -1;

However, some subdevice drivers don't handle it and may cause a problem
with the pxa_camera driver if the video device is opened by more than
two users at the same time.

Instead of propagating the boilerplate code for each subdevice driver
that implement s_power, this introduces an trick that many V4L2 drivers
are using with v4l2_fh_is_singular_file().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>