vp9-svc: Allow for adapt_rd_thresh with row-mt.

vp9-svc: Allow for adapt_rd_thresh with row-mt.

Set adaptive_row_thresh_mt = 1 at speed >= 7,
for svc when multi-threading is used with row-mt.
This allow the adaptive_rd_thresh feature to be used
in the nonrd-pickmode.

~1-2% speedup for SVC encoding with small quality
loss (< 0.6%) on RTC set.

Change-Id: Iab9878dff117bccdaef3e4d0645165db9808cdfc