drm/dp-mst: fix warning on unused var

Fixes: 83fa9842afe7 ("drm/dp-mst: Drop connection_mutex check")
Signed-off-by: Lucas De Marchi <[e-mail]>
Reviewed-by: José Roberto de Souza <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_dp_mst_topology.c

···
 struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
 								    struct drm_dp_mst_topology_mgr *mgr)
 {
-	struct drm_device *dev = mgr->dev;
-
 	return to_dp_mst_topology_state(drm_atomic_get_private_obj_state(state, &mgr->base));
 }
 EXPORT_SYMBOL(drm_atomic_get_mst_topology_state);