drm/i915/gt: Protect execlists_hold/unhold from new waiters

As we may add new waiters to a request as it is being run, we need to
mark the list iteration as being safe for concurrent addition.

v2: Mika spotted that we used the same trick for signalers_list, so warn
the compiler about the lockless walk there as well.

Fixes: 32ff621f ("drm/i915/gt: Allow temporary suspension of inflight requests")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200207110213.2734386-1-chris@chris-wilson.co.uk