atv: xorg.conf improvements

changes:

a) nvidia-auto-select can detect presence of TV-0 or DFP-0, but without TVStandard the TV-0 modepool does not contain 1080i or 720p modes and users probably see a black screen. TVStandard is ignored if DFP-0: is the active device

b) removing the extension section and Composite="false" as this has not been supported for some time