Make following Changes...Replace
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="tabcontainer" TargetControlID="btnSubmit"
CancelControlID="btnClose" BackgroundCssClass="modalBackground" >
</cc1:ModalPopupExtender>
tag with below tag
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnSubmit"
CancelControlID="btnClose" BackgroundCssClass="modalBackground" >
</cc1:ModalPopupExtender>
and remove style = "display:none" property from Panel1