drm/modeset_lock: document trylock_only in kerneldoc

drm/modeset_lock: document trylock_only in kerneldoc

I've forgotten to do this in:

commit ccf6cd9dd8e591a146c68b159e3f8521cdb1ee7d
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>