vp9: Avoid encoder loopfilter for non-reference frames.

vp9: Avoid encoder loopfilter for non-reference frames.

Useful for SVC, where the top layer enhancement frames may
not update any reference buffers, as is the case for the
patterns in the 1 pass CBR SVC when #temporal_layers > 1.

~3% encoder speedup for SVC patterns with temporal layers
in 1 pass CBR mode.

Updated the SVC datarate tests for the mismatch frames.
Set the frame-dropper off in some tests with #temporal_layers > 1
so we can correctly set #mismatch frames. Adjusted rate target
threshold for tests where frame-dropper was turned off.

Change-Id: Ia0c142f02100be0fed61cd2049691be9c59d6793