Which was a workaround to balance the quality between
luma and chroma when PVQ is enabled, by
1) skipping PVQ DC RDO for chroma and
2) using smaller lambda value (x0.5) for intra prediction of chroma,
intended to spend more bits and thus improve quality for chroma.

The change in metric for 5 frames of objective-1-fast is:
PSNR  PSNR-HVS  SSIM  CIEDE-2000  PSNR Cb PSNR Cr  MS-SSIM
-1.45 -1.54     -1.46 4.62        13.71   14.87     -1.47

For still images (subset1)
PSNR  PSNR-HVS  SSIM  CIEDE-2000  PSNR Cb PSNR Cr  MS-SSIM VMAF
-1.52 -1.54     -1.60 2.56        11.92   12.17     -1.52  -1.66

Change-Id: I31c4d7a7de2939f6004c112649f7303f0e944156