vp9-svc: Adjust some logic on gf temporal reference.

vp9-svc: Adjust some logic on gf temporal reference.

For the feature of using second temporal reference (when
inter-layer is off): move the buffer_idx assignement and
refresh flag settings further down to vp9_rc_get_svc_params(),
since is_key_frame is set there for every frame/layer.
Otherwise it was using the setting from the previous frame/layer.

This makes the refresh more consistent for both layers for
2 spatial layers case.

Small/negligible change in metrics.

Change-Id: I88279243bc27898448e8891dba38143d936cf6d5