vout/interlacing: don't inherit sout-deinterlace-mode

We just release the inherited value and overwrite a new value. This also fixes a crash if the software deinterlacer module is not found.

var_CreateGetString is just:

    var_Create( p_obj, psz_name, VLC_VAR_STRING | VLC_VAR_DOINHERIT );
    return var_GetString( p_obj, psz_name );