vp9-svc: Add condition on allocation for scaled_temp.

vp9-svc: Add condition on allocation for scaled_temp.

scaled_temp frame is used as an intermediate buffer for
2 stage down-sampling: two stages of 1/2 down-sampling
for a target of 1/4x1/4. This is used in 3 layer SVC
to avoid duplicate frame downsampling (on middle layer).

As this allocation is only needed/used when the
number_spatial_layers > 2, add this condition to avoid
unneeded allocation for 1 and 2 spatial SVC.

Change-Id: If342466644f685c1ea3ca5344b581793e5136c09