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

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -134,7 +134,7 @@ static struct sun4i_tcon *sun4i_get_tcon0(struct drm_device *drm)
 }
 
 void sun4i_tcon_set_mux(struct sun4i_tcon *tcon, int channel,
-			struct drm_encoder *encoder)
+			const struct drm_encoder *encoder)
 {
 	int ret = -ENOTSUPP;
 
@@ -783,7 +783,7 @@ static int sun4i_tcon_remove(struct platform_device *pdev)
 
 /* platform specific TCON muxing callbacks */
 static int sun5i_a13_tcon_set_mux(struct sun4i_tcon *tcon,
-				  struct drm_encoder *encoder)
+				  const struct drm_encoder *encoder)
 {
 	u32 val;
 
@@ -799,7 +799,7 @@ static int sun5i_a13_tcon_set_mux(struct sun4i_tcon *tcon,
 }
 
 static int sun6i_tcon_set_mux(struct sun4i_tcon *tcon,
-			      struct drm_encoder *encoder)
+			      const struct drm_encoder *encoder)
 {
 	struct sun4i_tcon *tcon0 = sun4i_get_tcon0(encoder->dev);
 	u32 shift;

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 {