added 8x8 based Rate estimation for dualpred case

This commmit added logic for MB using dual-pred to compute rate
estimation based on correct transform size. The section of code
was previously located under #if CONFIG_DUALPRED, that was made
to be working with T8x8 experiment at the same time.

Change-Id: Iebc2518c03f11378b9c2e72905520f088b54d5c0