st/mesa: allow multiple concurrent waiters in ClientWaitSync
st/mesa: allow multiple concurrent waiters in ClientWaitSync

so->fence can be unreferenced by one thread while another thread is
somewhere in ClientWaitSync and expecting so->fence to be non-NULL.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98172

Cc: 12.0 13.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler