gallium: Fix leak of currently bound UBOs at CSO context destruction.

Cc: mesa-stable
Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7695>
(cherry picked from commit daaf5f1d1868bebec7931a51753236a850ebbd24)
2 files changed