drm/i915: add ILK/SNB support to ivybridge_irq_handler

And then rename it to ironlake_irq_handler. Also move
ilk_gt_irq_handler up to avoid forward declarations.

In the previous patches I did small modifications to both
ironlake_irq_handler an ivybridge_irq_handler so they became very
similar functions. Now it should be very easy to verify that all we
need to add ILK/SNB support is to call ilk_gt_irq_handler, call
ilk_display_irq_handler and avoid reading pm_iir on gen 5.

v2: - Rebase due to changes on the previous patches
    - Move pm_iir to a tighter scope (Chris)
    - Change some Gen checks for readability

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>