[brazen1]

Set it to 24Hz, then 12bit will appear.  After you select it, change it back to 60Hz.  When you match refresh rates, it will auto change back to 24Hz/12bits.  If you ever reboot, it will revert back to 8bit using that driver.  An alternative is 385.28.  Many displays cannot handle 12bit properly and will introduce banding.  You might see better results using 8bit and only you will know.