drm/i915/breadcrumbs: Reduce missed-breadcrumb false positive rate

Change the on-cpu check to on-runqueue to catch if the waiter has been
woken (and reset its current_state back to TASK_UNINTERRUPTIBLE to
perform the seqno check) but is sleeping due to being preempted off the
cpu.

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/20181203113701.12106-1-chris@chris-wilson.co.uk