freedreno: add fd_ringbuffer_new_object()

Add new API for reusable "state objects" which can be re-used multiple
times.  Backend implementation for msm will follow.  (Probably not
needed to support this for any device that uses kgsl backend, since this
is mostly useful for a5xx+.)

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