drm/i915/gt: Mark legacy ring context as lost

When we reset the legacy ring context, due to potential corruption over
suspend/resume, remove the valid bit so that we avoid loading garbage.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201210080240.24529-1-chris@chris-wilson.co.uk