drm/i915: Take all modeset locks for DP MST hotplug

drm/i915: Take all modeset locks for DP MST hotplug

While auditing various users of the connector/encoder lists I realized
that the atomic code is a very prolific user of them. And it only ever
grabs the mode_config->connection_mutex, but not the
mode_config->mutex like all the other code walking encoder/connector
lists.

The problem is that we can't grab the mode_config.mutex late in atomic
code since that would lead to locking inversions. And we don't want to
grab it unconditionally like the legacy set_config modeset path since
that would render all the fine-grained locking moot.

Instead just grab more locks in the dp mst hotplug code. Note that
drm_connector_init (which is the one adding the connector to these
lists) already uses drm_modeset_lock_all.

The other reason for grabbing all locks is that the dpms off in the
unplug function amounts to a modeset, so better to take all required
locks for that.

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