When loaded data successfully, the 'onLoadSuccess' event fires. By using this event, you can get the first row and then call 'setValue' method to set the combogrid's value. Please try this code:
var cc = $('#cc');
cc.combogrid({
onLoadSuccess: function(data){
if (data.rows.length){
var opts = cc.combogrid('options');
cc.combogrid('setValue', data.rows[0][opts.idField])
}
}
})