drm/locking: fix drm_modeset_acquire_ctx kernel-doc

The stack_depot member was added without kernel-doc, leading to below
warning. Fix it.

./include/drm/drm_modeset_lock.h:74: warning: Function parameter or
member 'stack_depot' not described in 'drm_modeset_acquire_ctx'
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: cd06ab2f ("drm/locking: add backtrace for locking contended locks without backoff")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120094856.3004147-1-jani.nikula@intel.com

include/drm/drm_modeset_lock.h

@@ -34,6 +34,7 @@ struct drm_modeset_lock;
  * struct drm_modeset_acquire_ctx - locking context (see ww_acquire_ctx)
  * @ww_ctx: base acquire ctx
  * @contended: used internally for -EDEADLK handling
+ * @stack_depot: used internally for contention debugging
  * @locked: list of held locks
  * @trylock_only: trylock mode used in atomic contexts/panic notifiers
  * @interruptible: whether interruptible locking should be used.