drm/arc: Drop surplus connector registration

drm_connector_register does nothing before drm_dev_register(), it
is meant for hotpluggable connectors only. Same for the unregister side.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210112084358.2771527-6-daniel.vetter@ffwll.ch