drm/sun4i: tcon: Make tcon_set_mux mode argument const

The drm_display_mode pointer can be mark const, so let's do it.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b0cce5a43fc3b56953d21a54fc3c14672f755f42.1508231063.git-series.maxime.ripard@free-electrons.com
diff --git a/drivers/gpu/drm/sun4i/sun4i_tcon.h b/drivers/gpu/drm/sun4i/sun4i_tcon.h
index d9e1357..d81c6e2 100644
--- a/drivers/gpu/drm/sun4i/sun4i_tcon.h
+++ b/drivers/gpu/drm/sun4i/sun4i_tcon.h
@@ -152,7 +152,7 @@ struct sun4i_tcon_quirks {
 	bool	needs_de_be_mux; /* sun6i needs mux to select backend */
 
 	/* callback to handle tcon muxing options */
-	int	(*set_mux)(struct sun4i_tcon *, struct drm_encoder *);
+	int	(*set_mux)(struct sun4i_tcon *, const struct drm_encoder *);
 };
 
 struct sun4i_tcon {