drm/i915: Correctly refcount connectors in hw state readou

This was forgotten when adding the the refcounting to
drm_connector_state.

v2: Don't forget to unreference existing connectors. This isn't
relevant on driver load, but this code also runs on resume, and there
we already have an atomic state. Spotted by Chris Wilson.

Cc: Gabriel Feceoru <gabriel.feceoru@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Marius Vlad <marius.c.vlad@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Fixes: d2307dea

 ("drm/atomic: use connector references (v3)")
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462541943-19620-1-git-send-email-daniel.vetter@ffwll.ch