I'm using an R9 390 and have the exact same problem. For some reason on the 17.12.1 drivers makes it centered way off up and to the left on the screen. Catalyst used to have overscan/underscan to correct problems like this but the new scaling slider has never worked correctly for me until once today. Rolling back to 17.11.1 fixes the problem again, but I tried everything to fix it on the new drivers to no avail. I use a custom resolution of 1824x1026 so that the edges aren't cut off.
An interesting note is that with the scaling slider when I only have 1 monitor turned on I can correct the edges on 17.12.1, but when I turn on a tv I have plugged into my mobo hdmi port (1920x1080, Intel HD Graphics 530) it throws my main monitor (1824x1026 custom resolution) up and to the left no matter the scaling option selected. This problem does not happen on 17.11.1.