drm/i915/selftests: Restore previous heartbeat interval

Use the defaults we store on the engine when resetting the heartbeat as
we may have had to adjust it from the config value during initialisation.

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/20210204211303.21347-1-chris@chris-wilson.co.uk


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>