vp10: clip MVs before adding to find_ref_mvs() list.

This causes the output of find_ref_mvs() to always be unique or zero.
A nice side-effect of this is that it also causes the output of
find_ref_mvs_sub8x8() to be unique-or-zero, and it will not ignore
available candidate MVs under certain conditions.

See issue 1012.

Change-Id: If4792789cb7885dbc9db420001d95f9b91b63bfa