Mythbuntu 16.04, ATI Radeon, HDMI overscan

Hallo,

how can i disable the overscan with the standard radeon driver for my ATI?
with the old fgrlx and the amd util it was done after a few klicks.

Specs/coection:
Asus MB with AMD E-450 with integrated Radeon HD 6320 graphics card (Desktop set at 1920x1080 Full HD) --> HDMI --> Denon AVR (input signal is 1920x1080 Full HD) --> HDMI --> LG LCD TV (input signal is 1920x1080 Full HD)

... and no - i will not let the AVR nor the LCD do any up/down scale

when i set xrandr .... overscan off, the picture is to small (~ 1.5 cm on each side)
when i set my tv to "Just Scan", the picture will fit
... both are no solutions!

all my other devices coected with HDMI (including NB with Ubuntu 14.04) provide a perfect picture out of the box

thanks for any hints or a clean solution

greetz
Manuel