drm/atmel: Fixup drm_connector_/unplug/unregister/_all

Accidentally fell through the cracks in

commit 6c87e5c3
Author: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Date:   Wed Mar 23 11:42:54 2016 +0300

    drm: Rename drm_connector_unplug_all() to drm_connector_unregister_all()

despite that Boris acked that patch.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>