vp9: Skip computation of best_sad for newmv, unless needed.
For non-rd pickmode:
best_pred_sad, computed for NEWMV-last, is only used for
skipping golden non-zero modes. Add condition to avoid this
computation if not used (i.e, if golden nonzero modes are not used).
And remove code for computing best_pred_sad for NEWMV-golden,
since that sad is not used.
No change in behavior; small speed gain (~1%) for svc encodes.