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

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

@@ -602,6 +602,7 @@ static int ring_context_pin(struct intel_context *ce, void *unused)
 static void ring_context_reset(struct intel_context *ce)
 {
 	intel_ring_reset(ce->ring, ce->ring->emit);
+	clear_bit(CONTEXT_VALID_BIT, &ce->flags);
 }
 
 static const struct intel_context_ops ring_context_ops = {