panfrost: Free NIR of blit shaders

panfrost: Free NIR of blit shaders

After we compile from NIR to a native binary, we can throw away the NIR.

17.47KB leaked over 104 calls from:
    0xffff87dcafc3
      in ??
    _mesa_hash_table_init
      at ../src/util/hash_table.c:163
      in /home/alyssa/rockchip_dri.so
    _mesa_hash_table_create
      at ../src/util/hash_table.c:186
    nir_lower_vars_to_ssa_impl
      at ../src/compiler/nir/nir_lower_vars_to_ssa.c:717
      in /home/alyssa/rockchip_dri.so
    nir_lower_vars_to_ssa
      at ../src/compiler/nir/nir_lower_vars_to_ssa.c:817
    optimise_nir
      at ../src/panfrost/midgard/midgard_compile.c:504
      in /home/alyssa/rockchip_dri.so
    midgard_compile_shader_nir
      at ../src/panfrost/midgard/midgard_compile.c:2895
      in /home/alyssa/rockchip_dri.so
    panfrost_build_blit_shader
      at ../src/panfrost/lib/pan_blit.c:103
      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>