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

commit 01e3a8feb40e54b962a20fa7eb595c5efef5e109 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>