vp9: Adjust 16x16 splot threshold for variance partition

vp9: Adjust 16x16 splot threshold for variance partition

For speeds < 7, increase threshold that controls the split
of 16x16->8x8 blocks, for resolutions 720p and higher.

Minor change for speed 5 (since it uses reference partition scheme
which only uses variance partition as first step).
For speed 6: ~0.5% increase in avgPSNR/SSIM metrics on ytlvie set.
No change in speed.

Change-Id: I5126580973201538d8ca26a9256b93c4d11d685b