drm/i915: Warn when releasing a frontbuffer while in use

Let's scream if we are about to release a frontbuffer which
is still in use.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209021918.16234-3-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/display/intel_frontbuffer.c

@@ -224,6 +224,8 @@ static void frontbuffer_release(struct kref *ref)
 	struct drm_i915_gem_object *obj = front->obj;
 	struct i915_vma *vma;
 
+	drm_WARN_ON(obj->base.dev, atomic_read(&front->bits));
+
 	spin_lock(&obj->vma.lock);
 	for_each_ggtt_vma(vma, obj)
 		vma->display_alignment = I915_GTT_MIN_ALIGNMENT;