drm/xe: Annotate xe_mem_region::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: 0887a2e7

 ("drm/xe: Make xe_mem_region struct")
Cc: Oak Zeng <oak.zeng@intel.com>
Cc: Michael J. Ruhl <michael.j.ruhl@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/20240109112405.108136-3-thomas.hellstrom@linux.intel.com