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: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NIke Panhc <ike.pan@canonical.com>
Signed-off-by: NMatthew Garrett <matthew.garrett@nebula.com>

drivers/platform/x86/Kconfig

@@ -300,8 +300,10 @@ config IDEAPAD_LAPTOP
 	depends on RFKILL && INPUT
 	depends on SERIO_I8042
 	select INPUT_SPARSEKMAP
+	select BACKLIGHT_CLASS_DEVICE
 	help
-	  This is a driver for the rfkill switches on Lenovo IdeaPad netbooks.
+	  This is a driver for Lenovo IdeaPad netbooks contains drivers for
+	  rfkill switch, hotkey, fan control and backlight control.
 
 config THINKPAD_ACPI
 	tristate "ThinkPad ACPI Laptop Extras"