backlight: fix pwm_bl.c to notify platform code when suspending

When suspending, pwm-bl sets duty cycle to 0, and shuts down the pwm
device.

This patch ensure that the platform code is called before that
(through the notify callback_, leaving a chance for the platform code
to configure GPIOs (shutting off the backlight, for example), much like
it is done during normal operations.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>