drm: atmel_hldc: Use generic drm_connector_register_all() helper

This driver used to have its own implementation of connector_register_all()
which actually was taken as a prototype of drm_connector_register_all().

Now when drm_connector_register_all() exists reusing it here.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461068693-11260-3-git-send-email-abrodkin@synopsys.com