drm/modeset_lock: document trylock_only in kerneldoc

drm/modeset_lock: document trylock_only in kerneldoc

I've forgotten to do this in:

commit 5d49c5ec82f67ebc863b44c3d3efaa6aac5c0793
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: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>