drm/i915/engine: hide preempt_hang selftest member from kernel-doc

drivers/gpu/drm/i915/gt/intel_engine_types.h:293: warning: Function parameter or member 'preempt_hang' not described in 'intel_engine_execlists'
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/dafd771bb75cf14965dd3b666987c58a438de134.1683041799.git.jani.nikula@intel.com

drivers/gpu/drm/i915/gt/intel_engine_types.h

@@ -289,6 +289,7 @@ struct intel_engine_execlists {
 	 */
 	u8 csb_head;
 
+	/* private: selftest */
 	I915_SELFTEST_DECLARE(struct st_preempt_hang preempt_hang;)
 };