drm/i915: add quirk to invert brightness on eMachines G725

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=59628


Reported-by: Roland Gruber <post@rolandgruber.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>