winsys/radeon: reduce BO cache timeout

winsys/radeon: reduce BO cache timeout

1000 ms is an extreme value for typical interactive loads. A large
cache has some disadvantages. Search for reusable BOs can take a long
time and memory might get exhausted.

Let's be rather conservative and use half of the old value,
500ms. This is beneficial to some loads on my test system and there
are no regressions.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>