platform/loongarch: laptop: Get brightness setting from EC on probe

Previously during driver probe, 1 is unconditionally taken as current
brightness value and set to props.brightness, which will be considered
as the brightness before suspend and restored to EC on resume. Since a
brightness value of 1 almost never matches EC's state on coldboot (my
laptop's EC defaults to 80), this causes surprising changes of screen
brightness on the first time of resume after coldboot.

Let's get brightness from EC and take it as the current brightness on
probe of the laptop driver to avoid the surprising behavior. Tested on
TongFang L860-T2 Loongson-3A5000 laptop.

Cc: stable@vger.kernel.org
Fixes: 6246ed09 ("LoongArch: Add ACPI-based generic laptop driver")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>