Fix ATI overscan when using HDMI

I found that the Catalyst Control Center is really buggy on Server 2012 and constantly freezes (including 12.9 beta)

When using HDMI there is a 1" border around the side and usually the CCC would be used to change that, but in this case I was unable to do this.

Note that DB7E61ED-B1A8-4E61-A3E2-E89558AAD06E may be different.

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Video\{DB7E61ED-B1A8-4E61-A3E2-E89558AAD06E}\0000]
"DigitalHDTVDefaultUnderscan"=dword:00000000