(1) Remove one parameter of the function - single_newmv as all setups
related to this parameter can be done (mostly already have been done)
before calling into the function;
(2) Remove one unneccessary checking on the mbmi->ref_frame[1] as the
line of code right before it indicates it is always a compound ref
scenario;
(3) Replace all mbmi->ref_frame[] that have been declared as refs[].

BUG=aomedia:442
BUG=aomedia:447

Change-Id: I022e69d2711dd88086403cf38e65c462b28383ce