drm/xe: Replace usage of mem_type_to_tile

Currently mem_type_to_tile() is being used to access the tile's underlying
tile.mem.vram. However, this function makes the assumption that a mem_type
will only ever map to a single tile. Now that the TTM vram manager contains
a pointer to the memory_region, make use of this in xe_bo.c.

As such, introduce a helper function res_to_mem_region() to get the
ttm_vram_mgr->vram from the BO's resource, and use this to replace usage
of mem_type_to_tile().

xe_tile is still needed to choose the migration context, so this part is
unchanged. But as this is only renaming usage, function is renamed now to
mem_type_to_migrate().

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>