How to install AMD graphic drivers on Ubuntu 18.04

You should be able to use the open source drivers that come with your system. You may just need to use the common nomodeset fix:

How do I set 'nomodeset' after I've already installed Ubuntu?

Using the built in driver is usually sufficient for radeon cards.

If you have done other mucking around, or added the PPA for the NVIDIA drivers (sorry about that) you can clean up with the following:

sudo apt-add-repository -r ppa:graphics-drivers/ppa && \
sudo apt update && \
sudo apt upgrade && \
sudo apt autoremove && \
sudo apt autoclean

Now, add the AMD updates PPA and update:

sudo add-apt-repository ppa:oibaf/graphics-drivers
sudo apt-get update
sudo apt upgrade

Then reconfigure your packages to be safe:

sudo apt install --reinstall xserver-xorg-video-amdgpu
sudo dpkg --configure -a
sudo dpkg-reconfigure gdm3 ubuntu-session xserver-xorg-video-amdgpu

Finally, you can enable accelerated video:

sudo apt-get install mesa-vdpau-drivers

Then to test the VDPAU driver with mpv use:

mpv --hwdec=vdpau yourvideofile

Now reboot the computer and make sure it still works. It does? Good! Now reboot once again and see if you can remove the nomodeset boot option. If it will work without it, this might improve performance.

Finally, once you are logged into the GUI, go to Settings > Devices > Displays and make sure the resolution is properly set.

If performance is still not up to standards after all of the above, you can manually install the closed source driver as a last resort.

If you decide you would still like to use the proprietary driver, you will need to download and install it. According to the output you posted, you need to download this driver:

AMD Radeon™ HD 8670M Series GPU Drivers & Support

Click on the "Linux x86_64" box and download the zip file.

You can follow this guide or post any additional updates to your question and I will improve the answer accordingly.