Skip redundant motion search in 4x4 level rd loop

This commit makes the encoder to perform motion search only once
per reference frame type for each 4x4/4x8/8x4 block. For bus_cif
at 2000 kbps, the runtime goes from 253812ms -> 217817ms
(14% speed-up) for speed 0.

Change-Id: I5f17599ccc8cfaf93ccb4f98fcb6008af6d79e92