drm/i915/crt: Make sure the hotplug interrupt is enabled

After disabling the hotplug interrupts for VGA detection on Ironlake, be
sure to re-enable them again afterwards.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30378
Signed-off-by: Yuanhan Liu <yuanhan.liu@intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>