drm/i915: Introduce GEN2_IRQ_RESET/INIT

Unify the appearance of the gen2 irq code with the gen3+ code by
introducing the GEN2_IRQ_RESET/INIT macros.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170818183705.27850-6-ville.syrjala@linux.intel.com