drm/i915: s/INTEL_INFO(dev_priv)->gen/INTEL_GEN(dev_priv) in i915_irq

INTEL_GEN() appears to be the new way of doing these platform checks, so
convert this i915_irq.c too.

Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1500402480-30741-1-git-send-email-dhinakaran.pandiyan@intel.com