drm/i915: WARN_ON failed map_and_fenceable

I just noticed in our code we don't really check the assertion, and
given some of the code I am changing in this area, I feel a WARN is very
nice to have.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: s/&/&&/ to fix typo on the check.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3211,6 +3211,8 @@ search_free:
 	if (i915_is_ggtt(vm))
 		obj->map_and_fenceable = mappable && fenceable;
 
+	WARN_ON(map_and_fenceable && !obj->map_and_fenceable);
+
 	trace_i915_vma_bind(vma, map_and_fenceable);
 	i915_gem_verify_gtt(dev);
 	return 0;