vp9: Skip computation of best_sad for newmv, unless needed.

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.

Change-Id: Ic2cbdef6c4e9a233a57c0db0eeac8ad5fcead366