drm/i915: Taint the kernel on dumping the GEM ftrace buffer

As the ftrace buffer is single shot, once dumped it will not update. As
such, it only provides information for the first bug and all subsequent
bugs are noise. The goal of CI is to have zero bugs, so taint the kernel
causing CI to reboot the machine; fix the bug and move on.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191110185806.17413-13-chris@chris-wilson.co.uk