Conditionally drop cutting tails in lv-map coeff optimization

When the optimized non-zero tail is longer that 2, the likelihood
that forcing to drop all the tail coefficients would give better
rate-distortion optimization is much lower. This commit makes the
encoder to check such conditions and speed up the lv-map/multi
encoding speed.

Local test clips at high target bit-rate / quality show 20% overall
encoding speed up. The coding performance change for lowres is
0.014%.

Change-Id: Ifcf1d18e453a83110ca8be794aee26f3e0237605