Align rate-distortion cost metric for chroma compoments

This commit aligns the rate-distortion metrics for both luma and
chroma components in super transform rate-distortion optimization.
It improves the coding gains due to var-tx and supertx experiments
by 0.2% for high resolution test sets.

Change-Id: Ib89d99e29cb5ee27b1f867e301954d4164d8b364