drm/i915: Report back whether the irq was armed when adding the waiter

The important condition that we need to check after enabling the
interrupt for signaling is whether the request completed in the process
(and so we missed that interrupt). A large cost in enabling the
signaling (rather than waiters) is in waking up the auxiliary signaling
thread, but we only need to do so to catch that missed interrupt. If we
know we didn't miss any interrupts (because we didn't arm the interrupt)
then we can skip waking the auxiliary thread.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Mika Kuoppala <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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