drm: rcar-du: Replace direct DRM encoder access with cast macro

Add a new macro to downcast an rcar_du_encoder pointer to a drm_encoder
pointer and use it. This prepares for the replacement of the
rcar_drm_encoder encoder field with a drm_slave_encoder.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
diff --git a/drivers/gpu/drm/rcar-du/rcar_du_encoder.h b/drivers/gpu/drm/rcar-du/rcar_du_encoder.h
index 4b906b9..c6334b4 100644
--- a/drivers/gpu/drm/rcar-du/rcar_du_encoder.h
+++ b/drivers/gpu/drm/rcar-du/rcar_du_encoder.h
@@ -35,6 +35,8 @@
 #define to_rcar_encoder(e) \
 	container_of(e, struct rcar_du_encoder, encoder)
 
+#define rcar_encoder_to_drm_encoder(e)	(&(e)->encoder)
+
 struct rcar_du_connector {
 	struct drm_connector connector;
 	struct rcar_du_encoder *encoder;