drm/tegra: hub: Remove bogus connection mutex check

The Tegra DRM never actually took that lock, so the driver was broken
until generic locking was added in commit:

	commit b962a120
	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: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/hub.c

@@ -605,11 +605,8 @@ static struct tegra_display_hub_state *
 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);