Remove mi_grid_* structures.
mi_grid_* are arrays of pointer to pointer. They save the pointers that point to the MIs in cm->mi. But they are unnecessary and complicated. The original goal was to remove MODE_INFO_t copy. But with an extra MODE_INFO_t pointer inside MODE_INFO_t, same goal could be achieved. This commit totally removes the mi_grid_* structures. But there are still many dummy MODE_INFO_t inside cm->mi which are a waste of memory. Next commit will do on-demand MODE_INFO_t allocation in order to save these memories. Change-Id: I3a05cf1610679fed26e0b2eadd315a9ae91afdd6