freedreno/log: fix build error

freedreno/log: fix build error

It seems some versions of gcc are less clever about const initializers:

```
../src/gallium/drivers/freedreno/freedreno_log.c:58:33: error: initializer element is not constant
 const unsigned msgs_per_chunk = bo_size / sizeof(uint64_t);
                                 ^~~~~~~
```

See https://gitlab.freedesktop.org/mesa/mesa/-/issues/2713

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4390>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4390>