avcenc: Fix PSNR computation for 420 semi-planar output in test

PSNR computation for 420 Semi-planar recon was incorrect.

Bug: 173151765
Test: avcenc -i qcif.yuv -w 176 -h 144 --bitrate 512000 \
      -o out.h264 --recon_enable 1 --recon recon.yuv \
      --input_chroma_format YUV_420SP_VU --recon_chroma_format \
      YUV_420SP_VU --num_frames 4 --psnr 1

Change-Id: I8de628107894bfdef6027f865b235aa01349bc4d
1 file changed