ACPI video: correct error-handling code

backlight_device_register may return an ERR_PTR
value rather than a valid pointer.

Problem found by Julia Lawall, properly fixed by Zhang Rui.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Len Brown <len.brown@intel.com>