This commit enables a context based motion vector entropy coding
conditioned on dynamic reference motion vector list. This (along with
the previous CL) imporves the coding gains due to dynamic motion
vector referencing based entropy coding:
derf   0.1%
hevcmr 0.2%
stdhd  0.7%
hevchr 0.4%

No encoding time change was observed.

Change-Id: I179c723844079195f6952a12582996a3ca9e9914