drm: Roll out drm_for_each_connector more

Now that we also grab the connection_mutex and so fixed the race with
atomic modeset we can use the iterator there too.

The other special case is drm_connector_unplug_all which would have a
locking inversion with the sysfs store/show functions if we'd grab the
mode_config.mutex around the unplug. We could just grab
connection_mutex instead, but that's a bit too much a dirty trick for
my taste. Also it's only used by udl, which doesn't do any other kind
of connector hotplugging, so should be race-free. Hence just stick
with a comment for now.

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