vp9_pick_inter_mode(): minor optimizations

1. remove duplicate initialization to mbmi->interp_filter.
2. move mv clamping into ref_frame loop instead of mode checking loop.
3. move the check if last frame is same as golden frame earlier to
avoid initialization of Golden reference related variables.

Change-Id: Idf2d05e19e94a24f69cc289687869fc71d2ff289