Your code

public function validasi()
{
    $idgambar = $this->input->post('idgambar');
    $username = $this->input->post('username');
    $password = $this->input->post('password');

    $is_valid = $this->vote_model->validate($username, $password);

    if ($is_valid) {
        $data['tambah'] = "Maaf anda sudah pernah melakukan Voting.";
        return $this->load->view('pilih/coba', $data);
    }

    $data['tambah'] = 'failed login';
    $data['message_error'] = FALSE;

    return $this->load->view('pilih/coba', $data);
}


public function validate($username, $password)
{
    $this->db->where(array('nim' => $username, 'password' => $password));
    return $this->db->get('mahasiswa')->row(0);
}