drm/i915: Warn on if set frontbuffer return value is not NULL on release

i915_gem_object_set_frontbuffer returns set frontbuffer pointer.  When we
are releasing frontbuffer we are clearing the pointer from the object. Warn
on if return value is not null.

v3: Check i915_gem_object_set_frontbuffer return value separately
v2: Instead of ignoring do drm_WARN_ON

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230928082924.164720-1-jouni.hogander@intel.com

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

@@ -251,7 +251,7 @@ static void frontbuffer_retire(struct i915_active *ref)
 static void frontbuffer_release(struct kref *ref)
 	__releases(&intel_bo_to_i915(front->obj)->display.fb_tracking.lock)
 {
-	struct intel_frontbuffer *front =
+	struct intel_frontbuffer *ret, *front =
 		container_of(ref, typeof(*front), ref);
 	struct drm_i915_gem_object *obj = front->obj;
 
@@ -259,7 +259,8 @@ static void frontbuffer_release(struct kref *ref)
 
 	i915_ggtt_clear_scanout(obj);
 
-	i915_gem_object_set_frontbuffer(obj, NULL);
+	ret = i915_gem_object_set_frontbuffer(obj, NULL);
+	drm_WARN_ON(&intel_bo_to_i915(obj)->drm, ret);
 	spin_unlock(&intel_bo_to_i915(obj)->display.fb_tracking.lock);
 
 	i915_active_fini(&front->write);