drm/radeon: Take all modeset locks for DP MST hotplug

Similar with the i915 take all modeset locks for mst hotplug. This is
needed to make sure radeon holds both mode_config.mutex and
mode_config.connection_mutex when updating the connector_list, which
is the new (interim) locking regime we want for that.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>