I can work around it by setting 1080 as the primary resolution (hackish via CRU). The primary resolution won't be scaled, so when I change to 2160 then it works. However this is sub-optimal.
The problem remains that GPU scaling can't be disabled.
I have what seems to be the same problem.
I have an AMD Radeon R9 200 Series graphics card and Catalyst Control Center version 2015.0803.1559.26924, with HDMI connection to a Seiki SE39UY04 LED TV/monitor.
With Windows 8.1 Pro (64 bit) I could use CCC Desktop Properties to switch between 3840 x 2160 30p Hz and
HDTV 1080p 60p Hz, both showing full screen, and the monitor indicating the correct change in resolution. Presets also worked.
Now with Windows 10 Pro (64 bit) (free upgrade), CCC can switch to 3840 x 2160 30p Hz, but applying
HDTV 1080p 60p Hz reduces the desktop size to one fourth, in the middle of the screen, and the monitor does not notify a change of resolution (still 3840x2160@30Hz).
In Windows, going to Settings -> System -> Display -> Advanced display settings -> 1920 x 1080 -> Apply does the same thing.
It is possible to set the monitor to 1080p@60Hz with the following:
Settings -> System -> Display -> Advanced display settings -> Display adapter properties -> List All Modes -> 1920 by 1080, True Color (32 bit), 60 Hertz -> OK -> Apply
But about half an icon width of the desktop edges is past the edge of the screen. Either Windows Advanced Display Settings or CCC can return to 3840x2160@30Hz.
So it looks like the display driver can do 1080p @ 60 Hz almost correctly, but only by digging down to "List All Modes." Both Windows 10 "Advanced display settings" and Catalyst Control Center will change the desktop to the correct size in pixels, but do not tell the monitor to make the corresponding change in resolution.
The over scan problem at 1080p is fixed on Seiki 39" by going to (with the TV remote) Menu, enter "0-0-0-0" (goes to the factory settings menu), and in Overscan set H Ratio and V Ratio to 100. Still haven't found a fix for needing to go to "List All Modes" to set 1080p@60HZ.
Tried a fresh windows 10 install today. Problem still persists.