libvpx

Fix speed 7 and speed 12 for rt

A recent change has introduced big quality drops for speed 7 and 12
for --rt mode. The change reverted the big drop and improved quality
by 9.5% for speed 7 and 13.4% for speed 12.

Change-Id: I07b82e3bb6002a73af486a083458c88877bdad01