Refactor partition mode cost calculation
Refactor partition mode cost calculation

Compute the coding block partition mode cost as additional rdcost
to the cumulative rate-distortion cost from each coding block. This
changes the coding performance slightly due to the rounding error.
The compression performance change is neutral.

Change-Id: Ibdccae0e79263a0e70af7592a8cb11458d795f8d

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