Use the frame ID that indicates the frame display order to identify
whether two different reference frames exist for inter-coded frames.
If there is only one unique reference valid in the reference buffer,
there is no need to signal reference_mode. Instead, the decoder may
identify such scenario and set reference_mode to SINGLE_REFERENCE.

Change-Id: If7d374f5355f153c50b408be5a9956a833c976c3