drm: Constify drm_encoder_slave_funcs

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-11-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_encoder_slave.h b/include/drm/drm_encoder_slave.h
index 8b9cc36..82cdf61 100644
--- a/include/drm/drm_encoder_slave.h
+++ b/include/drm/drm_encoder_slave.h
@@ -95,7 +95,7 @@
 struct drm_encoder_slave {
 	struct drm_encoder base;
 
-	struct drm_encoder_slave_funcs *slave_funcs;
+	const struct drm_encoder_slave_funcs *slave_funcs;
 	void *slave_priv;
 	void *bus_priv;
 };