backlight: lp8788: Fully initialize backlight_properties during probe

[ Upstream commit 39234682 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>