drm/amd/display: Fix unit test failure

[Why]
Some of unit tests use large scaling ratio such that when we
 calculate optimal number of taps, max_taps is negative.
 Then in recent change, we changed max_taps to uint instead
 of int so now max_taps wraps and is positive.  This change
 changed the behaviour from returning back false to return
 true and breaks unit test check

[How]
Add check to prevent max_taps from wrapping and set to 0
 instead

Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/sspl/dc_spl.c

@@ -1026,12 +1026,18 @@ static bool spl_get_optimal_number_of_taps(
 			lb_config, &num_part_y, &num_part_c);
 	/* MAX_V_TAPS = MIN (NUM_LINES - MAX(CEILING(V_RATIO,1)-2, 0), 8) */
 	if (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert) > 2)
-		max_taps_y = num_part_y - (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert) - 2);
+		if ((spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert) - 2) > num_part_y)
+			max_taps_y = 0;
+		else
+			max_taps_y = num_part_y - (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert) - 2);
 	else
 		max_taps_y = num_part_y;
 
 	if (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert_c) > 2)
-		max_taps_c = num_part_c - (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert_c) - 2);
+		if ((spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert_c) - 2) > num_part_c)
+			max_taps_c = 0;
+		else
+			max_taps_c = num_part_c - (spl_fixpt_ceil(spl_scratch->scl_data.ratios.vert_c) - 2);
 	else
 		max_taps_c = num_part_c;