vp9-svc: Add a buffer_idx is_used parameter for SVC.

vp9-svc: Add a buffer_idx is_used parameter for SVC.

For the case where a second (long term) temoral reference is
used in the SVC: this additional parameter is to make sure the
buffer slot selected for this reference is available for usage,
i.e., it is never used for any of the 3 references set for the
fixed SVC patterns.

And some code cleanup (replace cpi->svc).

No change in behavior.

Change-Id: Icba46edfbbefb94d5ea8e2d5c24cccd85a406ee6