It seems to be easy to develop...
You need to hide panels which id is bigger then currently selected panel, and show panels which id's are smaller or equal to currently selected panel. Try below code:
int count = ddlvehicles.Count;
int cursel = ddlvehicles.SelectedValue;
for (int i = count; i >2 ; i--)
{
((Panel)mvbooking.Views[0].FindControl("Panel" + i)).Visible = (i<=cursel);
}