drm/i915/mst: use reference counted connectors. (v3)

Don't just free the connector when we get the destroy callback.

Drop a reference to it, and set it's mst_port to NULL so
no more mst work is done on it.

v2: core mst accepts NULLs fine. Cleanup EDID code properly.
v3: drop the extra reference we were taking.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>