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