drm: Amend connector list locking rules

Now that dp mst hotplug takes all locks we can amend the locking rules
for the iterators. This is needed before we can roll these out in the
atomic code to avoid getting burried in WARNINGs.

v2: Rebase onto the extracted list locking assert and add a comment to
explain the rules.

v3: Fixup German->English translation fail in the comment.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>