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
3 files changed