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':
ideapad-laptop.c:(.devexit.text+0x64a7): undefined reference to `backlight_device_unregister'
drivers/built-in.o: In function `ideapad_acpi_add':
ideapad-laptop.c:(.devinit.text+0x45e28): undefined reference to `backlight_device_register'

To select backlight subsystem can prevent this error from happening.

Also update comment for this driver.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>