iio: light: gp2ap002: Take runtime PM reference on light read

The light sensor needs the regulators to be enabled which means
the runtime PM needs to be on.  This only happened when the
proximity part of the chip was enabled.

As fallout from this change, only report changes to the prox
state in the interrupt handler when it is explicitly enabled.

Fixes: 97d642e2 ("iio: light: Add a driver for Sharp GP2AP002x00F")
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>