If you're open to using jQuery:
<script type="text/javascript">
function fncsave()
{
$('#<%= savebtn.ClientID %>').click();
}
</script>
Also, if you are using .NET 4 or better you can make the ClientIDMode == static
and simplify the code:
<script type="text/javascript">
function fncsave()
{
$("#savebtn").click();
}
</script>
Reference: MSDN Article for Control.ClientIDMode