vp9: Set low variance flag when partition is copied.

vp9: Set low variance flag when partition is copied.

Also set the flag to 1 when exit early choosing 64x64 block
such that skipping new mv for golden works in these scenerios.

Change the size of prev_segment_id to the number of superblocks
to save memory.

Borg test shows quality regression of 0.012% on average PSNR
and 0.035% on SSIM.

Change-Id: I5014224c8617d439d35c66ece3fed9ae30b31d23