Adding function build_inter_predictors_for_planes to build inter
predictors for specified planes. This function allows to remove
condition "#if CONFIG_ALPHA" and use MAX_MB_PLANE for general case.
Renaming 'which_mv' local var to 'ref', and 'weight' argument to 'ref'.

Change-Id: I1a97160c9263006929d38953f266bc68e9c56c7d