drm/xe: Rework size helper to be a little more correct

The _total_vram_size helper is device based and is not complete.

Teach the helper to be tile aware and add the ability to size
DG1 correctly.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>