Cleaning up vp9_mvref_common.{h, c}.

Hiding vp9_find_mv_refs_idx() inside vp9_mvref_common.c, moving definition
of vp9_find_mv_refs() to vp9_mvref_common.c.

Change-Id: I0c9f34b03648785a7d18edf6d4fddd34e55dfcc5
2 files changed