drm/amd/display: cleanup the code a bit

In function dc_sink_destruct, kfree will check pointer, no need
to check again.
This change is to cleanup the code a bit.
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_sink.c

@@ -33,14 +33,6 @@
  * Private functions
  ******************************************************************************/
 
-static void dc_sink_destruct(struct dc_sink *sink)
-{
-	if (sink->dc_container_id) {
-		kfree(sink->dc_container_id);
-		sink->dc_container_id = NULL;
-	}
-}
-
 static bool dc_sink_construct(struct dc_sink *sink, const struct dc_sink_init_data *init_params)
 {
 
@@ -75,7 +67,7 @@ void dc_sink_retain(struct dc_sink *sink)
 static void dc_sink_free(struct kref *kref)
 {
 	struct dc_sink *sink = container_of(kref, struct dc_sink, refcount);
-	dc_sink_destruct(sink);
+	kfree(sink->dc_container_id);
 	kfree(sink);
 }