This is probably already fixed in drm-next by
commit 0ad4dc887d4168448e8c801aa4edd8fe1e0bd534
Author: Hans de Goede <hdegoede@xxxxxxxxxx>
Date: Thu May 18 13:06:44 2017 +0200
drm/i915: Fix new -Wint-in-bool-context gcc compiler warning
> which I also think is a more sensible fix than this one.