[CFL] Replace DC_PRED magic numbers with base +/- 1
[CFL] Replace DC_PRED magic numbers with base +/- 1

This change does not impact the bitstream. It is intended to simplify
High bit depth support in subsequent commits.

Change-Id: If22822dbb3329fc0b4a5e343772d63761d46d3f1

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