CRM-13889 - api_v3_DashboardContact - Fix validation

The validation of dashboard_id implicitly involved a permission-check. This
permission-check would be wrong in some cases (eg running a drush script
without any particular user), but it could be right in other cases (eg
issuing an AJAX call).
Merge pull request #2466 from totten/4.4-dashboard-api-fix

CRM-13889 - api_v3_DashboardContact - Fix validation