pwm_backlight: remove unnecessary ifdefs

When the macro such as SIMPLE_DEV_PM_OPS is used, there is
no need to use '#ifdef CONFIG_PM' to prevent build error. Thus,
this patch removes unnecessary ifdefs.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>