Fix inconsistent use of ref_mv_idx.

"+ 1" has been added in a few locations to match the indices for the
context for the CDFs and the motion vectors.

Suggested fix (of removing +1s entirely) can then be implemented
without further affecting the overall behaviour of the encoder and
decoder.

BUG=aomedia:858

Change-Id: I650a618d3fda1d27bd3550d25ad1ac52e4365061