the tile boundary info flag was updated for each mi data structure
which starts from cm->mi. there is the second level buffer which
contains the pointer to the mi structure. it starts at
cm->mi_grid_visible. for every coded block, pointer in the second level
buffer only points to the top left mi structure of that particular coded
block. in order to get the correct boundary info for each mi. we have to
access the mi structure from cm->mi instead of cm->mi_grid_visible.
this change doesn't impact the result, it only cleans up the logic.

Change-Id: I8f883a284f600e3075754124b8197d78898a56be