[PATCH] ideapad: depends on backlight subsystem and update comment

I've got a report of build failure on ideapad-laptop, which shows drivers/built-in.o: In function `ideapad_acpi_notify': ideapad-laptop.c:(.text+0x63876a): undefined reference to `backlight_force_update' drivers/built-in.o: In function `ideapad_acpi_remove':

[PATCH] ideapad: depends on backlight subsystem and update comment

I've got a report of build failure on ideapad-laptop, which shows drivers/built-in.o: In function `ideapad_acpi_notify': ideapad-laptop.c:(.text+0x63876a): undefined reference to `backlight_force_update' drivers/built-in.o: In function `ideapad_acpi_remove':