drm/radeon: constify more struct drm_*_helper funcs pointers

Some non-const pointers were added since the last constification, fix
them.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/radeon/radeon_dp_mst.c b/drivers/gpu/drm/radeon/radeon_dp_mst.c
index 5952ff2b..1017338 100644
--- a/drivers/gpu/drm/radeon/radeon_dp_mst.c
+++ b/drivers/gpu/drm/radeon/radeon_dp_mst.c
@@ -604,7 +604,7 @@
 	struct radeon_encoder *radeon_encoder;
 	struct radeon_encoder_mst *mst_enc;
 	struct drm_encoder *encoder;
-	struct drm_connector_helper_funcs *connector_funcs = connector->base.helper_private;
+	const struct drm_connector_helper_funcs *connector_funcs = connector->base.helper_private;
 	struct drm_encoder *enc_master = connector_funcs->best_encoder(&connector->base);
 
 	DRM_DEBUG_KMS("enc master is %p\n", enc_master);