ACPI video: no warning message if "acpi_backlight=vendor" is used

AML code always sends notifications to ACPI video device,
even if we disable the ACPI backlight control by using
boot option "acpi_backlight=vendor".

In this case we should not print any warning message.
http://bugzilla.kernel.org/show_bug.cgi?id=13671#c14

Sigend-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>