I've find the solution. You need to deactivate the "Intel Virtualization Technology" in the BIOS and you will see your GPUs. 
Hi,I am trying to connect GPUs card to a Poweredge R720 through a C410x PCIe Expansion Chassis. After multiples try (BIOS upgrade, UEFI switching, multiple OS reinstall, disconnecting the power supply, BMC upgrade, iPass cable swaping, etc ...), I'm not able to view any GPU card in the pci device list ...The setup is very basic :C410x with GPUs <--> cable iPass  <--> PowerEdge R720 with HIC card Is there is specific configuration in the BIOS ? Something else ? 
PCI device - R720 & C410x