Non-rd partition: Use force split on 16x16 for low resolutions.

For low resolutions, whem 4x4downsample is used for variance,
use the same force split (that is used for 8x8downsample) for 16x16 blocks.

No change in metrics. Small improvement visually.

Change-Id: I915b9895902d0b9a41e75d37fee1bf3714d2366d