Periodically update mode and mv costs

Skip coefficient cost update in non-RD mode decision setting. Allow
periodical mode and motion vector cost update. Currently every other
8 frames. The increment runtime is a constant number. Hence more
visible for CIF resolution, while negligible for 1080p.

Speed -6 compression performance for rtc set is improved by 4.5%.

Change-Id: I27e0ad7c521fcc2af1d825582cbdd1a27ac4c323