drm/i915: document why dvo/sdvo/crt need a special dpms function

In the cloned case, changing just one output but keeping the other, the
pipe state won't change and intel_crtc_update_dpms will be a nop, but we
still need to update the dpms state of the output being changed.

Only dvo, sdvo and crt are cloneable, so only those three have special
dpms functions.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_dvo.c

@@ -180,6 +180,7 @@ static void intel_enable_dvo(struct intel_encoder *encoder)
 	intel_dvo->dev.dev_ops->dpms(&intel_dvo->dev, true);
 }
 
+/* Special dpms function to support cloning between dvo/sdvo/crt. */
 static void intel_dvo_dpms(struct drm_connector *connector, int mode)
 {
 	struct intel_dvo *intel_dvo = intel_attached_dvo(connector);
@@ -201,6 +202,8 @@ static void intel_dvo_dpms(struct drm_connector *connector, int mode)
 		return;
 	}
 
+	/* We call connector dpms manually below in case pipe dpms doesn't
+	 * change due to cloning. */
 	if (mode == DRM_MODE_DPMS_ON) {
 		intel_dvo->base.connectors_active = true;