platform/loongarch: laptop: Add backlight power control support

loongson_laptop_turn_{on,off}_backlight() are designed for controlling
the power of the backlight, but they aren't really used in the driver
previously.

Unify these two functions since they only differ in arguments passed to
ACPI method, and wire up loongson_laptop_backlight_update() to update
the power state of the backlight as well. Tested on the 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>