drm: Remove warning from drm_connector_unregister_all()

Commit 6c87e5c3 ("drm: Rename drm_connector_unplug_all() to
drm_connector_unregister_all()") replaced a manual connectors list walk
in drm_connector_unregister_all() with drm_for_each_connector(). The
list was walked without the mode config mutex locked as that ends up in
a clash with sysfs, but drm_connector_unregister_all() warns when the
mutex isn't locked.

The problem is known and doesn't require a large warning every time
drm_connector_unregister_all() is called. Fix it by reverting to manual
list walk.

Fixes: 6c87e5c3 ("drm: Rename drm_connector_unplug_all() to drm_connector_unregister_all()")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461190874-32674-1-git-send-email-laurent.pinchart+renesas@ideasonboard.com