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>