Though I don't have depth understanding of SDN but have basic knowledge on that.
SDN stands for software defined networking and it is application driven. Applications decide and configure the switches accordingly to make network faster and optimized.
When we talk about networking it means it is about Layer 2 and Layer 3 switches and software defined routing. Control plane and data plane both use this underlying facility of network switches and routers. In my understanding usage of SDN is independent from control and data plane. Both types of packets can use SDN but it is up to application which packets it wants to be controlled by SDN.
Mostly data path applications are using SDN to improve the throughput and performance. I never heard SDN is being used for the control plane packets.