drm/xe: Replace xe_ttm_vram_mgr.tile with xe_mem_region

Replace the xe_ttm_vram_mgr.tile pointer with a xe_mem_region pointer
instead. The former is currently unused.
TTM VRAM regions are exposing device vram and is better to store a pointer
directly to the xe_mem_region instead of the tile. This allows to cleanup
unnecessary usage of xe_tile from xe_bo.c in later patch.

Signed-off-by: Brian Welty <brian.welty@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>