Missing drivers for Radeon 8470D on 16.04

The proprietary driver fglrx is no longer supported by amd and you can't use it in ubuntu 16.04 (due to xorg and kernel versions). The alternative is the amdgpu opensource driver which your system will use it automatically if your gpu supported it. Since it doesn't seem to support it you only have the radeon opensource driver. If you need fglrx then yes, the only solution is installing ubuntu 14.04.
Some links to verify this (suggested by @Mark Kirby via comment) are here:

https://linux.slashdot.org/story/16/03/09/2049232/ubuntu-drops-support-for-amds-catalyst-gpu-driver

http://www.omgubuntu.co.uk/2016/03/ubuntu-drops-amd-catalyst-fglrx-driver-16-04