Some device families support a non-registered option for the 'ena' port of the ALTCLKCTRL megafunction, consult the respective device handbook for support of this feature.
The ALTCLKCTRL megafunction does not let you select a non-registered mode for the 'ena' port in the drop down menu. If you wish to use a non-registered mode for the 'ena' port, you can open the megafunction variation file and change the selection manually to "none".
For Verilog, locate the following in the defparam section:
sd1.ena_register_mode = "none"
For VHDL, locate the following in the GENERIC MAP section:
ena_register_mode => "none"