drm/bridge: tc358768: fix PLL parameters computation

According to Toshiba documentation the PLL input clock after the divider
should be not less than 4MHz, fix the PLL parameters computation
accordingly.

Fixes: ff1ca639 ("drm/bridge: Add tc358768 driver")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it

drivers/gpu/drm/bridge/tc358768.c

@@ -335,13 +335,17 @@ static int tc358768_calc_pll(struct tc358768_priv *priv,
 		u32 fbd;
 
 		for (fbd = 0; fbd < 512; ++fbd) {
-			u32 pll, diff;
+			u32 pll, diff, pll_in;
 
 			pll = (u32)div_u64((u64)refclk * (fbd + 1), divisor);
 
 			if (pll >= max_pll || pll < min_pll)
 				continue;
 
+			pll_in = (u32)div_u64((u64)refclk, prd + 1);
+			if (pll_in < 4000000)
+				continue;
+
 			diff = max(pll, target_pll) - min(pll, target_pll);
 
 			if (diff < best_diff) {