I don't have a PE6850 so I'm not familiar with the hardware at all.
The "user guide" does confirm that when an optional RAC is installed, the integrated video subsystem is disabled and the video controller on the RAC is used instead. The "system installation and trouble shoot guide" also states within the Video Connector section that installing a video card automatically disables the system's integrated video controller. This document also states within the Expansion card section that if you instal a RAC card, it must be installed in slot 7 (which is a PCI-X slot) to enable the card to be cabled to the RAC connector on the system board.
Thus it is not 100% clear if it is the act of installing a video card that disables the embedded video function or if it is a combination of the BIOS identifying the RAC card and connecting this card to the RMC connector on the motherboard that does the disabling deed. So you will just have to try out a video card and see.
If you have an spare PCIe video card lying about, you could modify it to allow installation in a 4x or 8x PCIe slot and see if this is indeed detected by BIOS, disables the embedded video function and is recognized by Windows 2012. Search the forums as i have posted such details on how to modify a x18 PCIe card to fit smaller x8 slots before. If you don't have a spare PCIe video card, they can be bought second hand for small change..
Alternatively you could use an old PCI video card and install it into the PCI-X slot, though you need to check that the card is indeed compatible with the PCI-X. Again if you don't have a PCI video card, they can be bought second hand for small change.
I'd go the PCIe route as you would be more likely to find drivers for Windows Server 2012 or for Win7 or for Win8 and there are more PCIe second hand cards about at very very cheap prices than old PCI/PCI-x video cards.
In any case it's a suck it and see approach so other than what i have said, i have no recommendations.