Clear {cb,cr}_grain_block when there is no scaling function
Clear {cb,cr}_grain_block when there is no scaling function

BUG=aomedia:1948,aomedia:1955

Change-Id: I0384e7ae9402f1117b97dae827097214e2907cbc

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom