drm/i915: enable vblank interrupt on ironlake

so far vblank interrupt on ironlake is disabled, this would cause
bad gfx performance if userspace calls drm_wait_vblank. This patch
enables vblank interrupt on ironlake and follows vblank get/put
model.

Signed-off-by: Li Peng <peng.li@intel.com>
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>