drm/sun4i: dsi: fix the overhead of the horizontal front porch

The formula in the BSP kernel indicates that a 16-byte overhead is used
when sending the HFP. However, this value is currently set to 6 in the
sun6i_mipi_dsi driver, which makes some panels flashing.

Fix this overhead value.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191006160303.24413-3-icenowy@aosc.io

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -569,11 +569,12 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 			  (mode->htotal - mode->hsync_end) * Bpp - HBP_PACKET_OVERHEAD);
 
 		/*
-		 * The frontporch is set using a blanking packet (4
-		 * bytes + payload + 2 bytes). Its minimal size is
-		 * therefore 6 bytes
+		 * The frontporch is set using a sync event (4 bytes)
+		 * and two blanking packets (each one is 4 bytes +
+		 * payload + 2 bytes). Its minimal size is therefore
+		 * 16 bytes
 		 */
-#define HFP_PACKET_OVERHEAD	6
+#define HFP_PACKET_OVERHEAD	16
 		hfp = max((unsigned int)HFP_PACKET_OVERHEAD,
 			  (mode->hsync_start - mode->hdisplay) * Bpp - HFP_PACKET_OVERHEAD);