drm/i915: panel: invert brightness via quirk

commit 4dca20efb1a9c2efefc28ad2867e5d6c3f5e1955 upstream.

A machine may need to invert the panel backlight brightness value. This
patch adds the infrastructure for a quirk to do so.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>