vp9-svc: Adjust logic on intra mode search.

vp9-svc: Adjust logic on intra mode search.

For SVC, on spatial enhancement layer, intra
search was disabled unless best reference frame
is golden (i.e., spatial/inter-layer prediction),
except for some other conditions (lower layer is key
or golden is not an allowed reference).

Fix is to add the base temporal layer condition,
so intra search will not be force-disabled for base
temporal layer frames.

This improves metrics (-1-2%) for SVC 3 and 2 layer config.
Some small encode time is expected, but since condition
only affect base temporal layers (i.e., every 4 frames
for 3 layers), increase is small.

Change-Id: I10b824faef99560dfdeeb02ba8bf8e3e1eea6255