I understand that only with Qt we can control DRM properties because it is the master and DRM can accepts only one master. But I don't need to implement any other drm interface, but I have to use qtplatform library, because they are speaking with drm.
So, investigating inside the code I have found a solution, but it use QPlatformScreen that is a Private Api:
add to cmake:
target_include_directories(appeturnix-qt6 PRIVATE
${qt_include_path}/QtGui/6.8.1/QtGui
)
and then you can use QPlatformScreen that is a private library...so...
QPlatformScreen *pScreen = screen->handle();
pScreen->setPowerState(QPlatformScreen::PowerStateOff);