drm/i915/selftests: Force ordering of context switches

The parallel switch test has an underlying assumption that its requests
are executed in order of submission, which is only true if the backend
manages to keep up. Ensure the order of execution matches the submission
order by explicit dependencies and so when we wait on the last request,
we know we wait on completion of the entire queue.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191016225730.29447-1-chris@chris-wilson.co.uk