add kqueue support to drm(4) by making the drm_sysfs_hotplug_event()

add kqueue support to drm(4) by making the drm_sysfs_hotplug_event()
available on OpenBSD well and by notifying listeners of a device
state change using EVFILT_DEVICE and NOTE_CHANGE.

drm_sysfs_hotplug_event() gets called when a state change of the device
occured, like an hdmi cable has been plugged, this in the future will
be used by the modesetting xorg driver to notify desktop environments
via randr events to update their screen configuration

ok kettenis@