vp9-svc: Add QP dependency to thresh_svc_skip_golden.

vp9-svc: Add QP dependency to thresh_svc_skip_golden.

In nonrd-pickmode: the golden/spatial reference for inter-layer
prediction may be skipped in the mode testing. Add QP dependency
to reduce the threshold for skipping (i.e., check it more often)
at high QP, if the lower layer was encoded at lower QP relative
to the current layer.

At high QP, a better quality lower resolution is more likely to
provide good spatial (inter-layer) prediction.

avgPSNR/SSIM metrics up by ~1% (all clips positive gain or neutral).
Some decrease in encode time (~1-2%) expected at lower bitrates,
for 3 layer SVC.

Change-Id: I9ee0f62d4b10d4ebd30165d378ecfa4399ae5ef1