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: Laktyushkin Dmytro <[e-mail]>
Acked-by: Solomon Chiu <[e-mail]>
Signed-off-by: Victor Lu <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 	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);