backlight: pwm_bl: Simplify usage of devm_gpiod_get_optional

Since 39b2bbe3

 (gpio: add flags argument to gpiod_get*() functions),
the gpiod_get* functions take an additional parameter that allows to
specify direction and initial value for output.
Simplify the usage of devm_gpiod_get_optional accordingly.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>