drm/vc4: Remove open-coded drm_connector_register_all()

drm_dev_register() will now register all known connectors, so we no
longer have to do so manually.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466501283-19976-3-git-send-email-chris@chris-wilson.co.uk