drm/i915: add quirk to enable backlight on Dell Chromebook 11 (2015)

Reported-by: Keith Webb <khwebb@gmail.com>
Suggested-by: Keith Webb <khwebb@gmail.com>
Cc: stable@vger.kernel.org
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=106671


Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446209424-28801-1-git-send-email-jani.nikula@intel.com