drm/i915: Drop bool return from breadcrumbs signaler

Since removal of the "missed interrupt detection" nobody used the result
of whether or not we signaled anybody during that invocation, so now
remove the return value.

References: 789659f4 ("drm/i915: Drop fake breadcrumb irq")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416085218.431-1-chris@chris-wilson.co.uk