Just an FYI, i have posted a bug report to the nouveau developers regarding the Linux kernels > 3.18 not being able to find the FCODE ROM / DCB block correctly. This seems to be an issue in getting newer kernels to work properly with G5 PowerMac machines (probably other models with nvidia gpu's) and OpenFirmware / FCODE ROM based nvidia adapters.
Last kernel I have packaged and run Xorg successfully without a workaround / hack is 3.18.16 and the most recent kernel i have tried is 4.1.2. Somewhere in between there was some regression.
Link Below;
https://bugs.freedesktop.org/show_bug.cgi?id=91319.