The VP9 style reference MV search(find_mv_refs_idx) exists in AV1,
and gather reference MVs in mv_ref_list, which is used to set
nearestmv and nearmv.

This patch switches the ref_mv search order, first call
setup_ref_mv_list() to find same reference frame MVs. If >= 2 MVs are
found, no more search is needed. Otherwise, we add MVs with different
reference frames. The purpose of doing this is to speed up the decoder.
Since we depends on setup_ref_mv_list() to find same reference frame
MVs, this change does change the bitstream. But, it wouldn't affect the
quality.

Borg test result:
        avg_psnr ovr_psnr  ssim
lowres:  -0.013  -0.016   -0.047

Change-Id: I219b9ca097b8fa90335d5b00f6edd639886f414d