pan/mdg: Free previous liveness

pan/mdg: Free previous liveness

Before we drop the reference.

160 calls with 0B peak consumption from:
    0xffffbd9d2fc3
      in ??
    pan_compute_liveness
      at ../src/panfrost/util/pan_liveness.c:127
      in /home/alyssa/rockchip_dri.so
    mir_compute_liveness
      at ../src/panfrost/midgard/midgard_liveness.c:55
      in /home/alyssa/rockchip_dri.so
    midgard_opt_dead_code_eliminate
      at ../src/panfrost/midgard/midgard_opt_dce.c:118
      in /home/alyssa/rockchip_dri.so

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Cc: mesa-stable
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6373>