Move the early exit checks on usable_ref_frame and
skip_ref_find_pref up before the check on flag_svc_subpel.
The code under flag_svc_subpel requires frame_mv to be set
for the golden/spatial reference, which is only set if the
both those exits don't pass.

No change in behavior.

Change-Id: Id304276c745eeb389ff85fa2dcf510d5976bc413