Hi Thomas,
Are you sure, you're testing with the latest drm-misc-next or drm-tip? Because using irq_enabled is deprecated and the flag was recently replaced by commit 1e4cd78ed493 ("drm: Don't test for IRQ support in VBLANK ioctls").
I was looking at drm-misc-fixes which did not have this commit :-( Just my silly excuse why I was convinced this was the issue.
Sam