vp9-svc: Fix to initialize downsampling filters.

vp9-svc: Fix to initialize downsampling filters.

For 3 spatial layers with 1/2 downsampling, the
downsampling filter for the middle layer was not
set for the very first frame, so it was defaulting
to the subsample filter (no averaging/phase = 0).

Its not set due to the two stage scaling that is
done for 1/4 on base layer, during which the intermediate
1/2 result is saved for the middle layer.

Fix for now is to set the default downsampling filter
to Bilinear (averaging/non-zero phase) for all layers on
init (vp9_init_layer_context):.

Change-Id: Ic7407810b34c621e7e7420682508d45478bdffcf