This commit generalizes dynamic reference motion vector coding mode
to support multiple candidate modes in the rate-distortion
optimization scheme and to support the selection in the bit-stream
syntax. The maximum number of modes allowed is currently limited to
4. The syntax elements for the dynamic reference motion vector
modes are using binary codes. The scheme supports single reference
frame.

It improves the compression performance
derf   0.135%
hevcmr 0.098%

Change-Id: Id053d6ce76e8365e52727bd0d12d28ce3de2e0e8