radeonsi: clear the debug callback on ctx destroy

The debug callback depends on the context to operate properly,
so we need to clear it on destroy to avoid a use-after-free.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12035
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32959>

src/gallium/drivers/radeonsi/si_pipe.c

@@ -194,6 +194,8 @@ static void si_destroy_context(struct pipe_context *context)
 {
    struct si_context *sctx = (struct si_context *)context;
 
+   context->set_debug_callback(context, NULL);
+
    util_unreference_framebuffer_state(&sctx->framebuffer.state);
    si_release_all_descriptors(sctx);