drm/i915: setup the hangcheck timer early

... together with all the other irq related resources in
intel_irq_init. I've managed to oops in the notify_ring function on my
ilk, presumably because of the powerctx setup call to i915_gpu_idle.

Note that this is only a problem with the reorder irq setup sequence
for irq-driver gmbus/dp aux.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>