libvpx

multi-res: add parent_ref_valid flag

Rather than overloading the parent_ref_frame value to shut off the
search in some cases, add a new validity flag. This cleans up some
of the duplicated mr_encoder_id && mr_low_res_mv_avail checks as
well, for readability.

Change-Id: Iddad93a27066c3d85ff2f25a361ac113b288ab7b