util/threaded_context: use driver's buffer alignment for staging transfers

this coincidentally worked because radeonsi has a hardcoded value of 64, but
other drivers do not use this value and then things are subtly broken

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7452>
1 file changed