Account sub8x8 block reference filter type for prob context

If a reference block is coded with sub8x8 block size, and if it
has sub-pixel level motion vectors, its prediction filter type
should be used as context information.

The coding performance gains of dual filter type coding scheme are
lowres  0.57%
hdres   0.88%

Change-Id: I68b98f2518d02f11c29d0256aeb45b2580fe5cac
6 files changed