JVET-N0448/N0380: MMVD fix when MaxNumMergeCand is 1
Implicitly infer the number of MMVD base merge candidate to be 1 as MaxNumMergeCand being equal to 1, and mmvd_merge_flag is not signalled in this case.
Implicitly infer the number of MMVD base merge candidate to be 1 as MaxNumMergeCand being equal to 1, and mmvd_merge_flag is not signalled in this case.