drm/sun4i: dsi: Change the start delay calculation

The current calculation for the video start delay in the current DSI driver
is that it is the total vertical size, minus the front porch and sync length,
plus 1. This equals to the active vertical size plus the back porch plus 1.

That 1 is coming in the Allwinner BSP from an variable that is set to 1.
However, if we look at the Allwinner BSP more closely, and especially in
the "legacy" code for the display (in drivers/video/sunxi/legacy/), we can
see that this variable is actually computed from the porches and the sync
minus 10, clamped between 8 and 100.

This fixes the start delay symptom we've seen on some panels (vblank
timeouts with vertical white stripes at the bottom of the panel).
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6e5f72e68f47ca0223877464bf12f0c3f3978de8.1549896081.git-series.maxime.ripard@bootlin.com

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -358,7 +358,9 @@ static void sun6i_dsi_inst_init(struct sun6i_dsi *dsi,
 static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi,
 					   struct drm_display_mode *mode)
 {
-	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1;
+	u16 start = clamp(mode->vtotal - mode->vdisplay - 10, 8, 100);
+
+	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
 }
 
 static void sun6i_dsi_setup_burst(struct sun6i_dsi *dsi,