st/pbo_compute: fix async->nir memory leak

This is an issue happening on radeonsi after the commit
6f8e6fb99c "mesa/st: use compute pbo download for readpixels".
This commit enables a new code path which has this memory leak.

For instance, this issue is triggered on radeonsi with
"piglit/bin/glsl-fs-raytrace-bug27060 -auto -fbo":
Too many leaks! Only the first 5000 leaks encountered will be reported.
Indirect leak of 327424 byte(s) in 5582 object(s) allocated from:
    #0 0x7fe27fa4d7ef in __interceptor_malloc (/usr/lib64/libasan.so.6+0xb17ef)
    #1 0x7fe2717fd4df in ralloc_size ../src/util/ralloc.c:118
    #2 0x7fe272dbbae4 in calc_dom_children ../src/compiler/nir/nir_dominance.c:138
    #3 0x7fe272dbbae4 in nir_calc_dominance_impl ../src/compiler/nir/nir_dominance.c:192
    #4 0x7fe272b7f4a2 in nir_metadata_require ../src/compiler/nir/nir_metadata.c:40
    #5 0x7fe272ba0d50 in nir_opt_cse_impl ../src/compiler/nir/nir_opt_cse.c:43
    #6 0x7fe272ba0d50 in nir_opt_cse ../src/compiler/nir/nir_opt_cse.c:67
    #7 0x7fe272686f83 in gl_nir_opts ../src/compiler/glsl/gl_nir_linker.c:92
    #8 0x7fe271a31e69 in create_conversion_shader ../src/mesa/state_tracker/st_pbo_compute.c:701
    #9 0x7fe271a353fa in create_conversion_shader_async ../src/mesa/state_tracker/st_pbo_compute.c:810
    #10 0x7fe271806ea8 in util_queue_thread_func ../src/util/u_queue.c:309
    #11 0x7fe27186379a in impl_thrd_routine ../src/c11/impl/threads_posix.c:67
    #12 0x7fe27ea9a7c3  (/lib64/libc.so.6+0x867c3)
...
SUMMARY: AddressSanitizer: 1384704 byte(s) leaked in 17291 allocation(s).

Fixes: 5dab7673e1 ("mesa/st: add specialized pbo download shaders")
Signed-off-by: Patrick Lerda <patrick9876@free.fr>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30068>

src/mesa/state_tracker/st_pbo_compute.c

@@ -1348,6 +1348,7 @@ st_pbo_compute_deinit(struct st_context *st)
          if (async->cs)
             st->pipe->delete_compute_state(st->pipe, async->cs);
          util_queue_fence_destroy(&async->fence);
+         ralloc_free(async->nir);
          ralloc_free(async->copy);
          set_foreach_remove(&async->specialized, se) {
             struct pbo_spec_async_data *spec = (void*)se->key;