ACPI video: Be more liberal in validating _BQC behaviour

Right now, if _BQC returns a value we don't understand we immediately
invalidate it. Change this behaviour so we only invalidate it if it
continues to give an invalid answer after we've already set a brightness.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>