drm/xe: Make xe_mem_region struct

Make a xe_mem_region structure which will be used in the
coming patches. The new structure is used in both xe device
level (xe->mem.vram) and xe_tile level (tile->vram).

Make the definition of xe_mem_region.dpa_base to be the DPA
base of this memory region and change codes according to
this new definition.

v1:
  - rename xe_mem_region.base to dpa_base per conversation with Mike
    Ruhl

Signed-off-by: Oak Zeng <oak.zeng@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>