[film grain] Fix clear of {cb,cr}_grain_block
[film grain] Fix clear of {cb,cr}_grain_block

BUG=aomedia:1995

Change-Id: Ied317364eba92a4bb903a42f87b870b7d719d93b

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