The control of using reference frame motion vector is a separate
factor from the existence of previous frame motion vectors. This
commit decouples these two, such that the encoder can control the
use of reference motion vector. When it is used, one can further
identify if the previous frame exists or not, then to decide if
need to force use_prev_frame_mvs to be zero.

This solves the issue where the previous frame mvs is set to be
0 and it accidentally shuts off the access to all other existing
referece frames mvs in the mfmv system. It brings back the coding
performance gains to normal.

Change-Id: I2531f73e55582a9bb5b3e0ff47e361a199ec8082