drm/i915: fix flip done interrupt on Ironlake

On Ironlake plane flip interrupt means flip done event already, the
behavior is not like old chips, and perform like other usual interrupt.
So only need to handle flip done event when receiving that interrupt.

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