drm/i915/selftests: Flush the preemption request before waiting

Make sure that the request has been submitted to HW before we begin our
wait. This reduces our reliance on the semaphore yield interrupt driving
the preemption request.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201222113536.3775-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/selftest_lrc.c

@@ -607,6 +607,10 @@ static int __live_lrc_gpr(struct intel_engine_cs *engine,
 		err = emit_semaphore_signal(engine->kernel_context, slot);
 		if (err)
 			goto err_rq;
+
+		err = wait_for_submit(engine, rq, HZ / 2);
+		if (err)
+			goto err_rq;
 	} else {
 		slot[0] = 1;
 		wmb();