drm/i915: Defer declaration of missed-interrupt until the waiter is asleep

drm/i915: Defer declaration of missed-interrupt until the waiter is asleep

If the waiter was currently running, assume it hasn't had a chance
to process the pending interrupt (e.g, low priority task on a loaded
system) and wait until it sleeps before declaring a missed interrupt.

References: https://bugs.freedesktop.org/show_bug.cgi?id=99816
Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Tvrtko Ursulin <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]