mesa/st: release variants for active programs before unref

mesa/st: release variants for active programs before unref

Programs can be shared among many contexts and each program holds a
variant list which has context specific variants. When context gets
destroyed it must make sure it relases all variants, otherwise remaining
context that utilizes same program will attempt to save a zombie shader
for already deleted context when releasing program and its variants.

Fixes:
   dEQP-EGL.functional.sharing.gles2.program.render

and other flaky multihread dEQP-EGL failures.

v2: pass program pointer via & (Marek)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4386>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4386>