The Tegra DRM never actually took that lock, so the driver was broken
until generic locking was added in commit:
	commit 
b962a12050a387e4bbf3a48745afe1d29d396b0d
	Author: Rob Clark <robdclark@gmail.com>
	Date:   Mon Oct 22 14:31:22 2018 +0200
	    drm/atomic: integrate modeset lock with private objects
It's now no longer necessary to take that lock, so drop the check.
v2: add rationale for why it is now safe to drop the check (Daniel)
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
 
 tegra_display_hub_get_state(struct tegra_display_hub *hub,
                            struct drm_atomic_state *state)
 {
-       struct drm_device *drm = dev_get_drvdata(hub->client.parent);
        struct drm_private_state *priv;
 
-       WARN_ON(!drm_modeset_is_locked(&drm->mode_config.connection_mutex));
-
        priv = drm_atomic_get_private_obj_state(state, &hub->base);
        if (IS_ERR(priv))
                return ERR_CAST(priv);