drm/modeset_lock: document trylock_only in kerneldoc

I've forgotten to do this in:

commit cb597bb3
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Jul 27 19:09:33 2014 +0200

    drm: trylock modest locking for fbdev panics

Oops, fix this asap.

In my defense kerneldoc is really awful and there's no way it can pick
up structured comments per struct member. Which means we need both
since people won't scroll up even a few lines.
Reviewed-by: NSean Paul <seanpaul@chromium.org>
Reviewed-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>

include/drm/drm_modeset_lock.h

@@ -33,6 +33,7 @@ struct drm_modeset_lock;
  * @ww_ctx: base acquire ctx
  * @contended: used internally for -EDEADLK handling
  * @locked: list of held locks
+ * @trylock_only: trylock mode used in atomic contexts/panic notifiers
  *
  * Each thread competing for a set of locks must use one acquire
  * ctx.  And if any lock fxn returns -EDEADLK, it must backoff and