This function will check if downgrading each coeff by one level
will reduce overall rd cost. If so, it will down grade the
coeff and update the effect and then move on to the next coeff.

In general, we found out that updating according to the
coding order will provide better coding performance.
The optimization order is as follows.
1) forward optimize coeffs == 1 or -1 only
2) backward optimize all coeffs

Change-Id: Ic0fd4d44d11878258e09d4fa87a8b48b397a10a8