Connecting Mac Pro with NVIDIA GeForce GTX 9XX to Apple Thunderbolt Display
In short, you can't.

This is because the Thunderbolt Display is a bit of a misnomer.  It's actually a Thunderbolt Dock with a display attached to it.

The requirements for the TB Display, per Apple are as follows:


  If your Mac has Thunderbolt ports, you can use a Thunderbolt display
  in macOS or Boot Camp. System requirements to use your Thunderbolt
  display in macOS: 
  
  
  A Thunderbolt-capable Mac
  The latest software and firmware updates for your Mac 
  OS X v10.6.8 Snow Leopard or later
  
  
  System requirements to use your Thunderbolt display in Boot Camp: 
  
  
  A Thunderbolt-capable Mac 
  The latest software and firmware updates for your Mac 
  OS X Lion: Boot Camp 4.0 is included with Lion OS X v10.6.8
  Snow Leopard: Boot Camp that’s updated to version 3.3
  
  
  (Emphasis mine)


If you look at the ports of the TB Display, you will notice there are
no "display" type ports (DP, mDP, DVI, HDMI, etc).  Just TB, USB,
FireWire and Network.  The Display adapter is actually "embedded" in
the display itself.



This is one of the reasons why you cannot connect an Apple Thunderbolt Display to a PC


  I figured I could simply adapt the larger format DisplayPort to the
  Mini-DisplayPort and get it to connect that way. That is assuming that
  the regular DisplayPort and Mini-DisplayPort are essentially the same
  connection/protocol with just a different interface size.


This is correct.  However, what you are missing that that the Thunderbolt Display doesn't accept DisplayPort or mini DisplayPort signaling.  Your source has to be Thunderbolt to begin with.