drm: Unexport drm_connector_unregister_all()

This has now been removed from all drivers as it is performed centrally
as a part of device unregistration for modesetting drivers. With the last
user gone, we can unexport it from the DRM module. That requires us to
move the code slightly to avoid the need for a forward declaration.

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
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468427947-28037-2-git-send-email-chris@chris-wilson.co.uk