drm/i915: unload: fix hotplug_work races

hotplug_work is queued by the hotplug interrupt and only either emits
a hotplug uevent or queues a crt poll slow-work. No other locking.  So
it's safe to cancel this work _after_ irq's have been turned off.  But
before the modesetting objects are destroyed because the hotplug
function accesses them (without locking).

The current code (for kms) only switches irqs off after modesetting
teardown, hence move the irq teardown into the modeset cleanup right
before the crtc cleanup.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>