vp9-svc: Fix in pickmode for key frames.

vp9-svc: Fix in pickmode for key frames.

For the fixed/default SVC patterns, GOLDEN is the
spatial reference, except on key frames, where LAST
is labeled as the spatial reference.

The current code was assuming GOLDEN is always the
spatial reference for the purpose of selecting the
subpel motion (due to the downsampling filter).

Fix is make sure flag_svc_subpel is set and used
with spatial_ref, which is labeled as the proper
spatial reference before entering mode check.

Some quality improvement on key frames.
Change-Id: Id236bcd47055b035731cc910ed84449d7e29f50c