make bsize requirement for SEG_LVL_SKIP explicit

The segment feature SEG_LVL_SKIP requires the prediction unit size
to be at least BLOCK_8X8. This commit makes the requirement to be
explicit. This is to prevent future encoder implementations from
making wrong choices.

Change-Id: I0127f0bd4c66e130b81f0cb0a8d3dbfe3b2da5c2