drm: Make the connector dpms callback return a value, v2.

This is required to properly handle failing dpms calls.
When making a wait in i915 interruptible, I've noticed
that the dpms sequence could fail with -ERESTARTSYS because
it was waiting interruptibly for flips. So from now on
allow drivers to fail in their connector dpms callback.

Encoder and crtc dpms callbacks are unaffected.

Changes since v1:
- Update kerneldoc for the drm helper functions.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Resolve conflicts due to different merge order.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dvo.c

@@ -197,7 +197,7 @@ static void intel_enable_dvo(struct intel_encoder *encoder)
 }
 
 /* Special dpms function to support cloning between dvo/sdvo/crt. */
-static void intel_dvo_dpms(struct drm_connector *connector, int mode)
+static int intel_dvo_dpms(struct drm_connector *connector, int mode)
 {
 	struct intel_dvo *intel_dvo = intel_attached_dvo(connector);
 	struct drm_crtc *crtc;
@@ -208,7 +208,7 @@ static void intel_dvo_dpms(struct drm_connector *connector, int mode)
 		mode = DRM_MODE_DPMS_OFF;
 
 	if (mode == connector->dpms)
-		return;
+		return 0;
 
 	connector->dpms = mode;
 
@@ -216,7 +216,7 @@ static void intel_dvo_dpms(struct drm_connector *connector, int mode)
 	crtc = intel_dvo->base.base.crtc;
 	if (!crtc) {
 		intel_dvo->base.connectors_active = false;
-		return;
+		return 0;
 	}
 
 	/* We call connector dpms manually below in case pipe dpms doesn't
@@ -238,6 +238,8 @@ static void intel_dvo_dpms(struct drm_connector *connector, int mode)
 	}
 
 	intel_modeset_check_state(connector->dev);
+
+	return 0;
 }
 
 static enum drm_mode_status