This patch abstracts the selection of the coefficient band
context into a function as a precursor to further experiments
with the coefficient context.

It also removes the large per TX size coefficient band structures
and uses a single matrix for all block sizes within the test function.

This may have an impact on quality (results to follow) but is only an
intermediate step in the process of redefining the context. Also the
quality impact will be larger initially because the default tables will
be out of step with the new banding.

In particular the 4x4 will in this case only use 7 bands. If needed we
can add back block size dependency localized within the function, but
this can follow on after the other changes to the definition of the
context.

Change-Id: Id7009c2f4f9bb1d02b861af85fd8223d4285bde5