cb4x4: Don't assume TX units are half MI_SIZE in skip context indexing.
Currently the "transform units" used to measure transform sizes in some parts of the code are based on the smallest defined transform size. cb4x4 currently defines a 2x2 transform size, even when chroma_2x2 is not enabled, which means that the scale of the transform units was always double that of MODEINFO units. Several areas of the code were hard-coding this assumption instead of converting from one to the other using appropriate constants. Change-Id: Ibc55671aa5bc3ad272cb8a036f9c4f9621df85ab