vp9: Speed feature to adapt partition based on source_sad.

vp9: Speed feature to adapt partition based on source_sad.

Move the source_sad feature to speed 6 (from speed 7), and
add speed feature to switch from the variance-based partition
to reference_partition (which uses nonrd-pickmode for bsize selection)
if source_sad is high.

Currently used only for speed 6 for resoln <= 360p.
About 4-5% improvement on 360p in RTC set.
Some speed slowdown, but still ~30% faster than speed 5.

Change-Id: Ib0330ee5fe9fdd2608aed91359a2a339d967491c