st/mesa: destroy only own program variants when program is released

st/mesa: destroy only own program variants when program is released

Earlier commit tried to achieve this but actually did more. This makes
sure the variants for other contexts continue to live.

Fixes: de3d7dbed52 ("mesa/st: release variants for active programs before unref")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2865
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4831>