drm/xe: Use __iomem for the regs pointer

drm/xe: Use __iomem for the regs pointer

The regs pointer points to IO memory. Annotate it properly and
fix the corresponding sparse warning.

Fixes: a4e2f3a299ea ("drm/xe: refactor xe_mmio_probe_tiles to support MMIO extension")
Cc: Koby Elbaz <kelbaz@habana.ai>
Cc: Ofir Bitton <obitton@habana.ai>
Cc: Moti Haimovski <mhaimovski@habana.ai>
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/20240109112405.108136-2-thomas.hellstrom@linux.intel.com