I didn't know that Ubuntu was using AMDGPU by default and I've installed fglrx. It's already removed now.
I want to reactivate AMDGPU, how can I do that?
lspci -nn | grep VGA
lsmod | grep amd
If your graphics card is supported you may wish to consider AMDGPU-PRO. You can find the latest 16.60 version of the driver here [link] (http://support.amd.com/en-us/kb-articles/Pages/AMDGPU-PRO-Driver-for-Linux-Release-Notes.aspx)