backlight: pwm_bl: Remove unneeded checks for valid GPIOs

All of gpiod_set_value_cansleep() and gpiod_direction_output() handle
NULL GPIO pointers just fine, so there is no need to check for that in
the caller.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/00be8237e0e2bc9b179177b5490f175d657261a2.1684856337.git.geert+renesas@glider.be


Signed-off-by: Lee Jones <lee@kernel.org>