pwm_backlight: avoid short blank screen while doing hibernation

Use SIMPLE_DEV_PM_OPS macro will initialize the member "freeze"
and "thaw" of pwm_backlight_pm_ops as below,
	.freeze = suspend_fn,
	.thaw = resume_fn,
then during the process of making hibernation snapshot, screen
will be blank at the moment of freezing, and then light at the
moment of thawing.
this is not the right user experience for suspending to disk.

so this patch drops freeze and thaw callback, make the LCD is
always lighting before the final shutdown.

Signed-off-by: Huayi Li <huayi.li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>