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: de3d7dbe ("mesa/st: release variants for active programs before unref") Closes: #2865 (closed) Cc: mesa-stable@lists.freedesktop.org Signed-off-by: Tapani Pälli tapani.palli@intel.com