Initialize mode cost using frame-level cdf.
Also in rd selection stage, cdf is updated per 64x64.
Performance gain 0.20%

Still suboptimal since in real bitstream packing, cdf is updated
per symbol. Per symbol update in RDO is work in progress.

Change-Id: I5062af91d8b00e5bf4c08abd0a7bfb0e5b27a619