freedreno: fix use-after-free with stateobj rb's

We could be dropping last reference in ->flush(), so clear the entry in
the parent rb's table to avoid deref'ing after free'd.

Also, ring_bo_del()'s use of ring_cache expects that it is dropping the
last reference.  So drop our ref to the stateobj's ring_bo first.

Signed-off-by: Rob Clark <robclark@freedesktop.org>