drm/sun4i: tcon: prevent tcon->panel dereference if NULL

If tcon->panel pointer is NULL, trying to dereference from it
(i.e. tcon->panel->connector) will cause a null pointer dereference.

Add tcon->panel null pointer check before calling
sun4i_tcon0_mode_set_dithering().

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Fixes: f11adcecbd ("drm/sun4i: tcon: Add dithering support for
                      RGB565/RGB666 LCD panels")
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005215951.99003-2-giulio.benetti@micronovasrl.com

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -491,7 +491,8 @@ static void sun4i_tcon0_mode_set_rgb(struct sun4i_tcon *tcon,
 	sun4i_tcon0_mode_set_common(tcon, mode);
 
 	/* Set dithering if needed */
-	sun4i_tcon0_mode_set_dithering(tcon, tcon->panel->connector);
+	if (tcon->panel)
+		sun4i_tcon0_mode_set_dithering(tcon, tcon->panel->connector);
 
 	/* Adjust clock delay */
 	clk_delay = sun4i_tcon_get_clk_delay(mode, 0);