Renaming of segment constants.

Renamed:
  MAX_MB_SEGMENTS to MAX_SEGMENTS
  MB_SEG_TREE_PROBS to SEG_TREE_PROBS

The minimum unit for segmentation in the segment map
is now 8x8 so it is misleading to use MB_ as macro-block
traditionally refers to a 16x16 region.

Change-Id: I0b55a6f0426bb46dd13435fcfa5bae0a30a7fa22