backlight: lp8788: Fully initialize backlight_properties during probe

props is stack allocated and the fields that are not explcitly set
by the probe function need to be zeroed or we'll get undefined behaviour
(especially so power/blank states)!

Fixes: c5a51053

 ("backlight: add new lp8788 backlight driver")
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240220153532.76613-4-daniel.thompson@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>