After some digging, found out it wasn't enabled by default in SCCM 2012.
To enable it, need to Go to Administration à Site Configuration à Client Setting à Default Client Settings (middle pane) à Properties
Under Default setting à Hardware Inventory à Set Classes.
Go under Logical Disk (SMS_LogicalDisk) à tick the column “Free Space (MB) à Press OK
Wait for sometimes for the hardware inventory cycle, then regenerate the report. The Free Disk Space data will be populated.
Isn't that hard, right?