rbug: Fix rbug_delete_vs_state lock acquisition.
Fix warning reported by Coverity Scan.
Double unlock (LOCK)
double_unlock: mtx_unlock unlocks rb_pipe->call_mutex while it is
unlocked.
Fixes: 07838ff990a7 ("rbug: Use the call mutex")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3023
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
cc: @wallbraker