How to display everytime Next and Previous button?

How i can display everytime next and previous?

When i'm in first page or last page the previous and next button disappear, i want display everytime the next and previous button.

Here is my source code for the <--next-page-->

add_filter('wp_link_pages_args', 'wp_link_pages_args_prevnext_add');
function wp_link_pages_args_prevnext_add($args)
{
    global $page, $numpages, $more, $pagenow;

    if (!$args['next_or_number'] == 'next_and_number') 
        return $args; # exit early

    $args['next_or_number'] = 'number'; # keep numbering for the main part
    if (!$more)
        return $args; # exit early

    if($page-1) # there is a previous page
        $args['before'] .= _wp_link_page($page-1)
            . $args['link_before']. $args['previouspagelink'] . $args['link_after'] . '</a>'
        ;

    if ($page<$numpages) # there is a next page
        $args['after'] = _wp_link_page($page+1)
            . $args['link_before'] . $args['nextpagelink'] . $args['link_after'] . '</a>'
            . $args['after']
        ;

    return $args;
}
function my_wp_link_pages( $args = '' ) {

    $r = wp_parse_args( $args, $defaults );
    $r = apply_filters( 'wp_link_pages_args', $r );
    extract( $r, EXTR_SKIP );

    global $page, $numpages, $multipage, $more, $pagenow;

    $output = '';
    if ( $multipage ) {
        if ( 'number' == $next_or_number ) {
            $output .= $before;
            for ( $i = 1; $i < ( $numpages + 1 ); $i = $i + 1 ) {
                $j = str_replace( '%', $i, $pagelink );
                $output .= ' ';
                if ( $i != $page || ( ( ! $more ) && ( $page == 1 ) ) )
                    $output .= _wp_link_page( $i );
                else
                    $output .= '<span class="current-post-page">';

                $output .= $text_before . $j . $text_after;
                if ( $i != $page || ( ( ! $more ) && ( $page == 1 ) ) )
                    $output .= '</a>';
                else
                    $output .= '</span>';
            }
            $output .= $after;
        } else {
            if ( $more ) {
                $output .= $before;
                $i = $page - 1;
                if ( $i && $more ) {
                    $output .= _wp_link_page( $i );
                    $output .= $text_before . $previouspagelink . $text_after . '</a>';
                }
                $i = $page + 1;
                if ( $i <= $numpages && $more ) {
                    $output .= _wp_link_page( $i );
                    $output .= $text_before . $nextpagelink . $text_after . '</a>';
                }
                $output .= $after;
            }
        }
    }

    if ( $echo )
        echo $output;

    return $output;
}