since:
92479d95 Make update_partition_context faster

fixes:
vp9/common/vp9_blockd.h:408:22: error:
non-constant-expression cannot be narrowed from type 'int' to 'char' in
initializer list [-Wc++11-narrowing]
  char pcvalue[2] = {~(0xe << boffset), ~(0xf <<boffset)};
                     ^~~~~~~~~~~~~~~~~

Change-Id: Id5b00b9a72d00a2b314081a23879bd1fa3ce983b