drm/i915: default to having backlight if VBT not available

Apparently there are Apple laptops with magic smoke for a VBIOS, which
we fail to find and use. Default to having and setting up backlight in
this case.

This fixes a regression introduced by
commit c675949e
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Wed Apr 9 11:31:37 2014 +0300

    drm/i915: do not setup backlight if not available according to VBT

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=77831


Reported-and-tested-by: Matteo Cypriani <mcy@lm7.fr>
Cc: stable@vger.kernel.org # 3.15+
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>