gpiolib: provide and use gpiod_line_state_notify()

Wrap the calls to blocking_notifier_call_chain() for the line state
notifier with a helper that allows us to use fewer lines of code and
simpler syntax.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>