Solved my own problem.. ROCm needs to be installed after the AMDGPU Blockchain drivers for Ubuntu... Happy Mining!
with latest AMDGPU-PRO v17.50 no need of ROCm...
I've installed a blank ubuntu 16.04 with amdgpu 17.50 --compute... it works perfectly.
but with previous version ROCm was solving DAG limitation (activate big 2M pages).