Poll, no poll, poll.

While working on gswitcheroo I needed a reliable way to determine if a monitor get's connected to the HDMI port of the discrete card. But monitoring the HDMI port when the corresponding discrete card is powered off, doesn't seem to work.
A temporary solution is to power on the discrete card once and again, to see if a some monitor got connected. airlied point out, that maybe the integrated card is able to detect connected monitors. But in the first trial it didn't really work.

What also didn't work, was monitoring status changes of the discrete (radeon) card using udev (using udevadm monitor).  Time for more investigations.