drm/xe: fix xe_mmio_total_vram_size

As also cause issue on PVC, moving back
to what we did before stolen was
introduced

Signed-off-by: Philippe Lecluse <philippe.lecluse@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>