drm/i915/selftests: Add intel_gt_suspend_prepare

Call suspend_prepare first so that we don't leave GuC so confused.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101174405.7389-1-chris@chris-wilson.co.uk

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

@@ -15,6 +15,7 @@ static int live_gt_resume(void *arg)
 
 	/* Do several suspend/resume cycles to check we don't explode! */
 	do {
+		intel_gt_suspend_prepare(gt);
 		intel_gt_suspend_late(gt);
 
 		if (gt->rc6.enabled) {