It is a refactoring patch, which aims to make the code ready for
implementation of in-frame mode cost update in RDO.
Also add mode cost update per sb row, but it won't affect coding
results because cdf update in RDO is not there.
Mode cost arrays are moved to MACROBLOCK because in multi-thread
coding, threads share the same AV1_COMP.

This patch does not have impact on coding results.

Change-Id: I2e8f7d7d066b23ebfbfc998269023781f359a6ff