Sir,
Please refer to the reference design, check fmc15x_dac.cpp, there we have implemented the complete init sequence. On of the last step in fmc15x_dac_init() is the data bus pattern check, we read address 0x8 and check for error. And this will only work after the clocking is correct.
Note that the chipset is not up and running (said differently cannot be used) before to actually run the software application and I assume this is what you are not doing.
I hope that helps,
Arnaud