drm: Add a callback from connector registering

If a driver wants to more precisely control its initialisation and in
particular, defer registering its interfaces with userspace until after
everything is setup, it also needs to defer registering the connectors.
As some devices need more work during registration, add a callback so
that drivers can do additional work if required for a connector.

Correspondingly, we also require an unregister callback.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[danvet: go ocd and remvoe unecessary empty kerneldoc line.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465993109-19523-3-git-send-email-chris@chris-wilson.co.uk