drm/i915/selftests: Restore to default heartbeat

Since we temporarily disable the heartbeat and restore back to the
default value, we can use the stored defaults on the engine and avoid
using a local.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200519063123.20673-3-chris@chris-wilson.co.uk