vp8: Modify to use closest reference in zero_mv bias.
Modify zero_mv bias condition to include check that "closest" reference is last_frame. This is needed for temporal layers, where the last_frame is not always the closest reference. Also, constain zeromv_count to be for last_frame reference. Change-Id: I7af54a809ebf01ef43b9933c9d4095b6cb189390