vp9-svc: Avoid minmax variance for non-reference frames.

vp9-svc: Avoid minmax variance for non-reference frames.

For choose_partitioning (speed >= 6): avoid computation
of minmax variance for non-reference frames in SVC.

Existing condition only avoided this for speed >= 8.
Combine that existing logic with non-reference condition.

Small speedup (~0.5-1%) for 3 layer SVC,
neutral change on avgPSNR/SSIM metrics.

Change-Id: I3e9f3a1af0647b15e475cf170d9402908d672ee5