util/threaded_context: use driver's ubo alignment for constant buffer uploads

this is another case where the hardcoded value was specific to radeon drivers

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7475>
2 files changed