drm/amd/display: Guard DST_Y_PREFETCH register overflow in DCN21

[why]
DST_Y_PREFETCH can overflow when DestinationLinesForPrefetch values are
too large due to the former being limited to 8 bits.

[how]
Set the maximum value of DestinationLinesForPrefetch to be 255 * refclk
period.
Reviewed-by: NLaktyushkin Dmytro <dmytro.laktyushkin@amd.com>
Acked-by: NSolomon Chiu <solomon.chiu@amd.com>
Signed-off-by: NVictor Lu <victorchengchi.lu@amd.com>
Tested-by: NDaniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dml/dcn21/display_mode_vba_21.c

@@ -841,6 +841,9 @@ static bool CalculatePrefetchSchedule(
 	else
 		*DestinationLinesForPrefetch = dst_y_prefetch_equ;
 
+	// Limit to prevent overflow in DST_Y_PREFETCH register
+	*DestinationLinesForPrefetch = dml_min(*DestinationLinesForPrefetch, 63.75);
+
 	dml_print("DML: VStartup: %d\n", VStartup);
 	dml_print("DML: TCalc: %f\n", TCalc);
 	dml_print("DML: TWait: %f\n", TWait);