drm/i915: add unregister callback to connector

Since

commit d9255d57
Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
Date:   Thu Sep 26 20:05:59 2013 -0300

it became clear that we need to separate the unload sequence into two
parts:

1. remove all interfaces through which new operations on some object
   (crtc, encoder, connector) can be started and make sure all pending
   operations are completed
2. do the actual tear down of the internal representation of the above
   objects

The above commit achieved this separation for connectors by splitting
out the sysfs removal part from the connector's destroy callback and
doing this removal before calling drm_mode_config_cleanup() which does
the actual tear-down of all the drm objects.

Since we'll have to customize the interface removal part for different
types of connectors in the upcoming patches, add a new unregister
callback and move the interface removal part to it.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>