drm/atomic: Add missing ERR_PTR casting

This is an oversight from

commit f52b69f1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Nov 19 18:38:08 2014 +0100

    drm/atomic: Don't overrun the connector array when hotplugging

Cc: Dave Airlie <airlied@redhat.com>
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>