Rework pred pixel buffer system in non-RD coding mode

This commit makes the inter prediction buffer system to support
hybrid partition search. It reduces the runtime of speed -5 by
about 3%. No compression performance change.

vidyo1 720p 1000 kbps
11831 ms -> 11497 ms

nik 720p 1000 kbps
10919 ms -> 10645 ms

Change-Id: I5b2da747c6395c253cd074d3907f5402e1840c36
Refactor vp9_update_rd_thresh_fact

Reduce the scope of function parameters.

Change-Id: Ifef2cfb559908a97498ffdbd6ea53da1cd45a73c
Fix mode index use case in vp9_pick_inter_mode

This improves coding performance of speed -5 and -6 by 0.6%,
respectively.

Change-Id: Ic5a7746a88c73285f0b14333d35dc16b02152c25
Merge "Rework pred pixel buffer system in non-RD coding mode"
Merge "Refactor vp9_update_rd_thresh_fact"
Merge "Fix mode index use case in vp9_pick_inter_mode"