drm/xe: Annotate multiple mmio pointers with __iomem

There are a couple of pointers pointing to MMIO space. Annotate them
with __iomem and fix the corresponding sparse warnings.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Fixes: 3b0d4a55 ("drm/xe: Move register MMIO into xe_tile")
Fixes: 399a1332

 ("drm/xe: add 28-bit address support in struct xe_reg")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Koby Elbaz <kelbaz@habana.ai>
Cc: Ofir Bitton <obitton@habana.ai>
Cc: Moti Haimovski <mhaimovski@habana.ai>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240109112405.108136-4-thomas.hellstrom@linux.intel.com