av1_predict_intra_block_facade: const correctness.
av1_predict_intra_block_facade: const correctness.

BUG=aomedia:448

Change-Id: Idc76c954435517a50a9cfa8e54fb73a593aaaac6

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