[PATCH] ideapad: Check if acpi already handle backlight power to avoid a page fault

This patch avoid a page fault in the ideapad-laptop extras when
turning the backlight power on or off.
Signed-off-by: Rene Bolldorf <xsecute@googlemail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/platform/x86/ideapad-laptop.c

@@ -493,6 +493,8 @@ static void ideapad_backlight_notify_power(struct ideapad_private *priv)
 	unsigned long power;
 	struct backlight_device *blightdev = priv->blightdev;
 
+	if (!blightdev)
+		return;
 	if (read_ec_data(ideapad_handle, 0x18, &power))
 		return;
 	blightdev->props.power = power ? FB_BLANK_UNBLANK : FB_BLANK_POWERDOWN;