drm/i915: add ironlake_irq_reset

To merge the common code of ironlake_irq_preinstall and
ironlake_irq_uninstall.

We should also probably do something about that HSWSTAM write on a
later commit.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Fix compile fail due to drm_i915_private_t typedef removal.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>