vp9-svc: Fix on disabling inter_layer prediction.

vp9-svc: Fix on disabling inter_layer prediction.

In vp9_svc_constrain_inter_layer_pred() we disable the
inter_layer prediction if anything but only the previous
spatial layer (from same supeframe) is used for inter_layer
prediction. This check and disabling was only allowed when
the control VP9E_SET_SVC_INTER_LAYER_PRED is set to
INTER_LAYER_PRED_ON_CONSTRAINED.

But the control VP9E_SET_SVC_INTER_LAYER_PRED is needed for setting:
INTER_LAYER_PRED_ON/INTER_LAYER_PRED_OFF/INTER_LAYER_PRED_OFF_NONKEY.
So there is a conflict with setting INTER_LAYER_PRED_ON_CONSTRAINED.

Fix for now is to always allow for this disabling check
(disable inter_layer reference if its not previous spatial layer) as
long as inter_layer prediction is used (i.e., not set to _OFF).

A separate fix if needed may be to invoke another control for setting
INTER_LAYER_PRED_ON_CONSTRAINED.

This was causing an issue with enabling spatial layers on the fly
(say spatial layer 2), where since INTER_LAYER_PRED_ON_CONSTRAINED was
not set (default), the inter_layer prediction was then using a reference
from 2 spatial layers below (spatial layer 0).

Change-Id: Ic6434000665f63aab27c509b5eb7b8fc965827bc