drm/i915/selftests: Flush the active barriers before asserting

Before we peek at the barrier status for an assert, first serialise with
its callbacks so that we see a stable value.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728153325.28351-1-chris@chris-wilson.co.uk

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

@@ -68,6 +68,8 @@ static int context_sync(struct intel_context *ce)
 	} while (!err);
 	mutex_unlock(&tl->mutex);
 
+	/* Wait for all barriers to complete (remote CPU) before we check */
+	i915_active_unlock_wait(&ce->active);
 	return err;
 }