drm/i915: Drop racy markup of missed-irqs from idle-worker

During the idle-worker we disable the hangcheck and so kick any waiters
that should have been completed (since the GPU is now idle). Unlike the
hangcheck, we do not take any care to avoid the race between the irq
handler and ourselves, and so it is possible for us to declare a missed
interrupt even as the bottom-half is being scheduled to run. Let's
ignore this race to stop a potential false-positive error.

References: https://bugs.freedesktop.org/show_bug.cgi?id=96974
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469351421-13493-1-git-send-email-chris@chris-wilson.co.uk