vp9-svc: Modify logic for frame dropping with spatial layers.

vp9-svc: Modify logic for frame dropping with spatial layers.

In the constrained framedrop mode for svc: modify the buffer check
condition relative to (non-zero) dropmark to include uppper spatial layers,
in addition to the current spatial layer.

But keep the single layer check if the buffer goes below zero, since
in this case (buffer underflow) we should force drop of that layer
regardless of upper layers.

Change-Id: Id277f0b4a3ae6275effdd5f5f0c80e3229c17424