Enable skipping reference frame check in rd loop

Enable skipping reference frame check in rd loop

This commit allows encoder to compare the SAD cost associated with
the best motion vector predictor, per frame. If one reference frame
has this cost more than 4 times of the best SAD cost given by other
reference frames, skip NEARESTMV, NEARMV, ZEROMV mode check of this
reference frame.

This setting is turned on in speed 2 and above. Compression quality
change in speed 2:
derf  -0.014%
yt    -0.097%
hd    -0.023%
stdhd  0.046%

It reduces the speed 2 runtime of test sequences:
pedestrian_area_1080p 4000 kbps 310763 ms -> 303595 ms
bluesky_1080p 6000 kbps         259852 ms -> 251920 ms

Change-Id: I7f59cf79503d51836d61d56d50dc5bdf0e502e22