freedreno/msm: remove reset of linked rings

The msm_cmd isn't refcount'd, so with stateobj rb's that have
independent lifecycle, this is no longer a safe thing to do.
Really, now that there is a bo-cache for rb's, fd_ringbuffer_reset()
should be deprecated because it adds a bunch of pointless complexity.

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