In the decoder, the function get_tx_size_context() is called
by read_selected_tx_size(), *before* mbmi->ref_frame is set up.
This means that mbmi->ref_frame still holds the ref frames from
the previous block, and does not indicate if the current block is
inter or not.

Fortunately, all of the other functions called by read_selected_tx_size()
have an explicit "is_inter" parameter, which is set correctly.
So we can just plumb this through into get_tx_size_context().

Change-Id: I00f0f3704937f592ccc5d304ed20ee25196dd520