drm/xe: Annotate xe_ttm_stolen_mgr::mapping with __iomem

The pointer points to IO memory, but the __iomem annotation was
incorrectly placed. Annotate it correctly, update its usage accordingly
and fix the corresponding sparse error.

Fixes: d8b52a02

 ("drm/xe: Implement stolen memory.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240109112405.108136-5-thomas.hellstrom@linux.intel.com