There is this nice (internal) class in Qt for Embedded Linux called QWSDisplay. In particular, it contains a static method called:
static void QWSDisplay::setTransformation(int transformation, int screenNo = -1);

It can be found in src/gui/embedded/qwsdisplay_qws.h

In order to work, Qt needs to have the Transformed gfx plugin available. The function above is the only way to achieve rotation of the screen, but the class is internal, but this function is static, so it's not too dangerous. Many customers ask for a way to programmatically rotate the embedded device screen, and they can't find any way to do it in the documentation.