Otherwise anv_descriptor_set is accessed through an unaligned pointer,
which is undefined behavior in C.
```
anv_descriptor_set.c:1620:17: runtime error: member access within misaligned address 0x61900002c2b5
for type 'struct anv_descriptor_set', which requires 8 byte alignment 0x61900002c2b5
```
Fixes: 2570a58bcd ("anv: Implement descriptor pools")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32070>
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