drm/sun4i: tcon: Make tcon_get_clk_delay 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/e6f92f126640aa6de639386f9b4677db3d8bb37b.1508231063.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -146,7 +146,7 @@ void sun4i_tcon_set_mux(struct sun4i_tcon *tcon, int channel,
 }
 EXPORT_SYMBOL(sun4i_tcon_set_mux);
 
-static int sun4i_tcon_get_clk_delay(struct drm_display_mode *mode,
+static int sun4i_tcon_get_clk_delay(const struct drm_display_mode *mode,
 				    int channel)
 {
 	int delay = mode->vtotal - mode->vdisplay;