AMDGPU-pro 18.50 opencl pal and legacy drivers are recognized as different platforms, but with Windows 19.3.2 adrenalin drivers and ROCm 2.2 drivers these cards are correctly assigned to one AMD platform. for the RX480 it uses the legacy drivers, and for Vega 56 it uses pal drivers.
pal.legacy.txt is the output of clinfo AMDGPU-pro 18.50 opencl pal and legacy drivers
win.txt is the output of clinfo in windows
rocm.txt is the output of clinfo with ROCm 2.2 drivers
The issue is fixed by installing mesa drivers, provided by Padoka's PPA, on top of AMDGPU-pro drivers, then clinfo throws 3 platforms, 1 for Vega and polaris for mesa drivers, 1 for polaris for pro drivers, 1 for vega for pro drivers. Somehow opencl applications are able to use the platform provided by mesa drivers but actually using pro drivers for each device.
I thought the solution was for all applications, it only works for blender with cycles, with radeon pro render the installer crashes, with luxrender you have to manually set the devices, in libreoffice it crashes.