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

commit d4afc775 upstream.

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>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Tested-by: Artem X <artem.brz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>