vp9-svc: Fix to SVC for frame dropping.

vp9-svc: Fix to SVC for frame dropping.

When the previous frame is dropped, for the current
spatial layer make sure the lst_fb_idx corresponds
to the buffer index last updated on the (last) encoded
TL0 frame(for same spatial layer).

This is needed to preserve the temporal prediction pattern
for fixed/non-flexible mode under frame dropping.

Change-Id: Ifc8e257beb025654a81580c4da0a181235724508