Download AGDPfix 1.3
This is a simple Applescript app that will back up the AppleGraphicsControl.kex to your desktop in a folder called KextBackUp. It will then patch the /System/Library/Extensions/AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/Info.plist and change "Config1" to "none" for the MacPro6,1 board ID. This ensures that your GPU will not be effected by the MacPro6,1's AMD FirePro device policy that causes your GPU not to send a signal to your monitor at boot. This app is only useful if you are using the MacPro6,1 system definition.
Credit to PikeRAlpha:
What's New in Version 1.3 (See full changelog)
- Added support for iMac15/17 (config2), SIP check and caches rebuild.