drm: Unexport drm_connector_register_all()

This has now been removed from all drivers as it is performed centrally
as a part of device registration for modesetting drivers. With the last
user gone, we can unexport it from the DRM module.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
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/1466778982-6974-1-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1466778982-6974-2-git-send-email-chris@chris-wilson.co.uk