drm/bridge: tc358767: fix max_tu_symbol value

max_tu_symbol was programmed to TU_SIZE_RECOMMENDED - 1, which is not
what the spec says. The spec says:

roundup ((input active video bandwidth in bytes/output active video
bandwidth in bytes) * tu_size)

It is not quite clear what the above means, but calculating
max_tu_symbol = (input Bps / output Bps) * tu_size seems to work and
fixes the issues seen.

This fixes artifacts in some videomodes (e.g. 1024x768@60 on 2-lanes &
1.62Gbps was pretty bad for me).
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Tested-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190924131702.9988-1-tomi.valkeinen@ti.com

drivers/gpu/drm/bridge/tc358767.c

@@ -728,6 +728,8 @@ static int tc_set_video_mode(struct tc_data *tc,
 	int lower_margin = mode->vsync_start - mode->vdisplay;
 	int vsync_len = mode->vsync_end - mode->vsync_start;
 	u32 dp0_syncval;
+	u32 bits_per_pixel = 24;
+	u32 in_bw, out_bw;
 
 	/*
 	 * Recommended maximum number of symbols transferred in a transfer unit:
@@ -735,7 +737,10 @@ static int tc_set_video_mode(struct tc_data *tc,
 	 *              (output active video bandwidth in bytes))
 	 * Must be less than tu_size.
 	 */
-	max_tu_symbol = TU_SIZE_RECOMMENDED - 1;
+
+	in_bw = mode->clock * bits_per_pixel / 8;
+	out_bw = tc->link.base.num_lanes * tc->link.base.rate;
+	max_tu_symbol = DIV_ROUND_UP(in_bw * TU_SIZE_RECOMMENDED, out_bw);
 
 	dev_dbg(tc->dev, "set mode %dx%d\n",
 		mode->hdisplay, mode->vdisplay);