drm/i915: safeguard against too high minimum brightness

drm/i915: safeguard against too high minimum brightness

Never trust (your interpretation of) the VBT. Regression from

commit 6dda730e55f412a6dfb181cae6784822ba463847
Author: Jani Nikula <jani.nikula@intel.com>
Date:   Tue Jun 24 18:27:40 2014 +0300

    drm/i915: respect the VBT minimum backlight brightness

causing div by zero if VBT minimum brightness equals maximum brightness.

Despite my attempts I've failed in my detective work to figure out what
the root cause is. This is not the real fix, but we have to do
something.

Reported-by: Mike Auty <mike.auty@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=86551
Cc: stable@vger.kernel.org (v3.17+)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>