drm/xe: Add missing doc for xe parameter

Fix the following warning:

	../drivers/gpu/drm/xe/xe_ttm_stolen_mgr.c:55: warning: Function
	parameter or member 'xe' not described in
	'xe_ttm_stolen_cpu_inaccessible'
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/xe/xe_ttm_stolen_mgr.c

@@ -41,6 +41,7 @@ to_stolen_mgr(struct ttm_resource_manager *man)
 /**
  * xe_ttm_stolen_cpu_inaccessible - Can we directly CPU access stolen memory for
  * this device.
+ * @xe: xe device
  *
  * On some integrated platforms we can't directly access stolen via the CPU
  * (like some normal system memory).  Also on small-bar systems for discrete,