drm/xe: Fix MTL+ stolen memory mapping

Based on commit 8d8d062b ("drm/i915/mtl: Fix MTL stolen memory GGTT
mapping"). For stolen on MTL and beyond, the address in the PTE is the
offset from DSM base. While at it, update the comments explaining each
part of the calculation.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20230726160708.3967790-9-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>