I found some indications here : http://wiki.cchtml.com/index.php/Configuring
I followed those steps :
1) aticonfig --set-pcs-val=MCIL,DigitalHDTVDefaultUnderscan,0
2) aticonfig --query-dispattrib=tmds2i #to see supported values
3) aticonfig --set-dispattrib=tmds2i,sizeX:1920 # to set X resolution
4) aticonfig --set-dispattrib=tmds2i,sizey:1080 # to set Y resolution
5) aticonfig --set-dispattrib=tmds2i,positionX:0 # to set X position to 0
6) aticonfig --set-dispattrib=tmds2i,positionX:0 # to set Y position to 0
Unfortunaly, after those steps, the image is too large!!!
Then, I tried to reload xserver (CTRL+ALT+TAB) and I never managed to launch X... I couldn't even get console mode by using CTRL+ALT+F1...
Moreover, I used a ubuntu live CD to edit my xorg.conf file, but even after that, it's impossible to launch X on my TV.
On the link below, it is said
Is there any chance that's why I can't start X again?
This command will disable underscan and saves the option until next driver update