Use modeled rate distortion costs for non-RD mode

This commit replaces SAD cost with modeled rate-distortion cost
for non-RD mode decision. It translates the prediction residual
SSE into estimate rate and reconstruction distorion costs, hence
capturing the quantization setting effect. The compression
performance of speed -7 for rtc set is improved by 14.79%.

Change-Id: Ifda014eb0501d13109fe7f92680bf1410b463632