Skip UV intrabc processing for monochrome
Skip UV intrabc processing for monochrome

BUG=aomedia:1580

Change-Id: I87bd2ba9607f0e7109caf5330b5b85f13beea407

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