drm/xe: Limit drm_mm_node_allocated access to xe_ggtt_node

Continue with the encapsulation of drm_mm_node inside xe_ggtt.

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821193842.352557-7-rodrigo.vivi@intel.com


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>