drm/dp_mst: Add __must_check to drm_dp_mst_topology_mgr_resume()

Since I've had to fix two cases of drivers not checking the return code
from this function, let's make the compiler complain so this doesn't
come up again in the future.

Changes since v1:
* Remove unneeded __must_check in function declaration - danvet
Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Jerry Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108211133.32564-4-lyude@redhat.com

include/drm/drm_dp_mst_helper.h

@@ -616,7 +616,8 @@ void drm_dp_mst_dump_topology(struct seq_file *m,
 			      struct drm_dp_mst_topology_mgr *mgr);
 
 void drm_dp_mst_topology_mgr_suspend(struct drm_dp_mst_topology_mgr *mgr);
-int drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
+int __must_check
+drm_dp_mst_topology_mgr_resume(struct drm_dp_mst_topology_mgr *mgr);
 struct drm_dp_mst_topology_state *drm_atomic_get_mst_topology_state(struct drm_atomic_state *state,
 								    struct drm_dp_mst_topology_mgr *mgr);
 int drm_dp_atomic_find_vcpi_slots(struct drm_atomic_state *state,