Refactor sub8x8 tx size RD for daala-dist

For a tx size RD search with partition size >= 8x8 and tx size < 8x8,
daala-dist function is applied to the whole partition after all tx blocks are encoded
instead of each 8x8 sub block of the partition.

Change-Id: I27d9e2960aa641f550096e32ebcdf8dfb4de79a6