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>

drivers/gpu/drm/drm_atomic.c

@@ -318,7 +318,7 @@ drm_atomic_get_connector_state(struct drm_atomic_state *state,
 	 */
 	if (index >= state->num_connector) {
 		DRM_DEBUG_KMS("Hot-added connector would overflow state array, restarting\n");
-		return -EAGAIN;
+		return ERR_PTR(-EAGAIN);
 	}
 
 	if (state->connector_states[index])