vp9: Skip some modes when variance is low for big blocks, for 1 pass real-time.

vp9: Skip some modes when variance is low for big blocks, for 1 pass real-time.

Skip intra-mode and some inter-modes (newmv, nearmv, nearestmv) for
golden frame if the variance got from choose_partitioning is very low.
Only for 1 pass real-time CBR mode and bsize >= 32x32, it has ~2.5%
speed up with less than 0.1% PSNR drop for rtc test set. Don't see
visual regression.

Change-Id: I70efbc95a1007231ae36f02c5b2fbf6cd35077ad