drm/i915: All members of gen4 have hotplug, so unconditionally enable its irq

Also as we set the HOTPLUG_EN to 0 during pre-install, we can simply set
it during post-install, and nor do we wish to enable unwanted hotplug
events.

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