xf86: Disable unused crtc functions when a lease is revoked
This fixes 'non-desktop' displays staying powered on after their lease has been revoked.
This is my first X change, so let me know if I did anything silly.
I'm also no expert on the lease/modeset architecture, so this might not be the right approach. But it seems to me like X is the one responsible for fixing the modes/powering down the displays at lease revokation time.