drm/amd/display: Explicitly cast v_total to signed in a subtraction

[WHY & HOW]
v_total is an uint32_t and subtracting an unsigned to a signed will
result in an unsigned which is always >= 0. As a result, the ternary
conditions are always true and thus has no effect.

This is fixed by casting v_total to signed explicitly. This also
avoids v_total subtraction to overflow.

This fixes 1 NO_EFFECT, 2 DEADCODE and 2 INTEGER_OVERFLOW issues reported
by Coverity.
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Alex Hung <alex.hung@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/core/dc_stream.c

@@ -1006,9 +1006,9 @@ static unsigned int dc_stream_get_max_flickerless_instant_vtotal_delta(struct dc
 	int safe_refresh_v_total = (int)div64_s64((long long)stream->timing.pix_clk_100hz*100, safe_refresh_hz*stream->timing.h_total);
 
 	if (increase)
-		return ((stream->timing.v_total - safe_refresh_v_total) >= 0) ? (stream->timing.v_total - safe_refresh_v_total) : 0;
+		return (((int) stream->timing.v_total - safe_refresh_v_total) >= 0) ? (stream->timing.v_total - safe_refresh_v_total) : 0;
 
-	return ((safe_refresh_v_total - stream->timing.v_total) >= 0) ? (safe_refresh_v_total - stream->timing.v_total) : 0;
+	return ((safe_refresh_v_total - (int) stream->timing.v_total) >= 0) ? (safe_refresh_v_total - stream->timing.v_total) : 0;
 }
 
 /*