drm/i915: add quirk to invert brightness on eMachines e725

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=31522#c35


[Note: There are more than one broken setups in the bug. This fixes one.]
Reported-by: Martins <andrissr@inbox.lv>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>