In the bidirectional case, it was possible to end up with
cm->ref_frame_idx_0 > cm->ref_frame_idx_1 (see linked bug report),
leading to av1_ref_frame_type() producing bad outputs.
This commit enforces the correct ordering.

BUG=aomedia:1336

Change-Id: I45db51064cd465907412b4fb301b7c17db5100af