drm/sun4i: dsi: Enforce boundaries on the start delay

The Allwinner BSP makes sure that we don't end up with a null start delay
or with a delay larger than vtotal.

The former condition is likely to happen now with the reworked start delay,
so make sure we enforce the same boundaries.
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c9889cf5f7a3d101ef380905900b45a182596f56.1549896081.git-series.maxime.ripard@bootlin.com

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -359,8 +359,12 @@ static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi,
 					   struct drm_display_mode *mode)
 {
 	u16 start = clamp(mode->vtotal - mode->vdisplay - 10, 8, 100);
+	u16 delay = mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
 
-	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
+	if (delay > mode->vtotal)
+		delay = delay % mode->vtotal;
+
+	return max_t(u16, delay, 1);
 }
 
 static void sun6i_dsi_setup_burst(struct sun6i_dsi *dsi,