drm/probe-helper: Fix drm_kms_helper_poll_enable() docs

Fix docs to reflect code and drm_kms_helper_poll_disable() docs by saying
that calling drm_kms_helper_poll_enable() is fine even if output polling
is not enabled.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106191812.38927-2-noralf@tronnes.org

drivers/gpu/drm/drm_probe_helper.c

@@ -216,8 +216,7 @@ enum drm_mode_status drm_connector_mode_valid(struct drm_connector *connector,
  * suspend/resume.
  *
  * Drivers can call this helper from their device resume implementation. It is
- * an error to call this when the output polling support has not yet been set
- * up.
+ * not an error to call this even when output polling isn't enabled.
  *
  * Note that calls to enable and disable polling must be strictly ordered, which
  * is automatically the case when they're only call from suspend/resume