drm: allow changing DPMS mode

The drm_atomic_helper_connector_dpms
will set the connector back the old DPMS state
before returning. This makes it impossible to change
DPMS state of the connector.

Fixes: 0853695c
v2: edit of commit message
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: dri-devel@lists.freedesktop.org
Cc: <drm-intel-fixes@lists.freedesktop.org>
Signed-off-by: Marta Lofstedt <marta.lofstedt@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161205120408.13056-1-marta.lofstedt@intel.com

drivers/gpu/drm/drm_atomic_helper.c

@@ -2885,8 +2885,8 @@ int drm_atomic_helper_connector_dpms(struct drm_connector *connector,
 fail:
 	if (ret == -EDEADLK)
 		goto backoff;
-
-	connector->dpms = old_mode;
+	if (ret != 0)
+		connector->dpms = old_mode;
 	drm_atomic_state_put(state);
 	return ret;