Before this patch, the functions build_inter_predictors, allow_warp
and av1_make_inter_predictor took two parameters (if
CONFIG_MOTION_VAR): mi_col_offset and mi_row_offset. These were used
for two things: they were used to look up a MODE_INFO structure and
the code figured out it was doing OBMC if at least one offset was
nonzero.

This patch slightly simplifies things by passing the relevant mi and a
flag saying whether we're doing OBMC instead. This means the
allow_warp function actually loses a parameter.

It also moves all mention of the functions out of the header and into
reconinter.c since they are only used in that compilation unit.

Change-Id: I8dedad9d7384ee0aa17b977e92ea7bfa83a532ef