drm: Avoid connector reference imbalance on error path

Whilst looking at the fallout from using connector references for
atomic, I noticed that there is an early return buried in
drm_atomic_set_crtc_for_connector() that if hit could cause us to leak a
reference on the connector.

Fixes: d2307dea14 (drm/atomic: use connector references (v3))
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Stone <daniels@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1462535265-13058-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_atomic.c

@@ -1160,14 +1160,18 @@ drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
 {
 	struct drm_crtc_state *crtc_state;
 
-	if (crtc)
-		drm_connector_reference(conn_state->connector);
-	if (conn_state->crtc && conn_state->crtc != crtc) {
+	if (conn_state->crtc == crtc)
+		return 0;
+
+	if (conn_state->crtc) {
 		crtc_state = drm_atomic_get_existing_crtc_state(conn_state->state,
 								conn_state->crtc);
 
 		crtc_state->connector_mask &=
 			~(1 << drm_connector_index(conn_state->connector));
+
+		drm_connector_unreference(conn_state->connector);
+		conn_state->crtc = NULL;
 	}
 
 	if (crtc) {
@@ -1177,18 +1181,16 @@ drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
 
 		crtc_state->connector_mask |=
 			1 << drm_connector_index(conn_state->connector);
-	}
 
-	if (conn_state->crtc)
-		drm_connector_unreference(conn_state->connector);
-	conn_state->crtc = crtc;
+		drm_connector_reference(conn_state->connector);
+		conn_state->crtc = crtc;
 
-	if (crtc)
 		DRM_DEBUG_ATOMIC("Link connector state %p to [CRTC:%d:%s]\n",
 				 conn_state, crtc->base.id, crtc->name);
-	else
+	} else {
 		DRM_DEBUG_ATOMIC("Link connector state %p to [NOCRTC]\n",
 				 conn_state);
+	}
 
 	return 0;
 }