This patch will make sure the use of the BWDREF_FRAME for the
encoding of both the two types of bipredictive frames, namely
LAST_BIPRED_UPDATE and BIPRED_UPDATE. To realize it, the
updates on the cpi->ref_frame_flags have been moved to before
the encoding of one frame, instread of originally handled after
the encoding of one frame.

RD performance has been improved slightly, approximately by 0.17%
compared to before the applying of this patch:

lowres: Avg -3.474; BDRate -3.324
derflr: Avg -2.097; BDRate -1.353

Change-Id: I0aa19afd752293e345489fbff104c4351ca5498c