anv/allocator: Use VG_NOACCESS_WRITE in anv_bo_pool_free

anv/allocator: Use VG_NOACCESS_WRITE in anv_bo_pool_free

Previously, we were relying on the fact that VALGRIND_MEMPOOL_FREE came
later on in the function to prevent "link->bo = bo" from causing an invalid
write.  However, in the case where the size requested by the user is very
small (less than sizeof(struct anv_bo)), this isn't sufficient.  Instead,
we should call VALGRIND_MEMPOOL_FREE early and then use VG_NOACCESS_WRITE.
We do, however, have to call VALGRIND_MEMPOOL_FREE after reading bo_in
because it may be stored in the bo itself.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>