st/mesa: destroy pipe_context before destroying st_context (v2)

st/mesa: destroy pipe_context before destroying st_context (v2)

If radeonsi starts compiling an optimized shader variant asynchronously
with a GL debug callback set and the application destroys the GL context,
radeonsi crashes when trying to write shader stats into the debug output
of a non-existent context after compilation, because st/mesa was destroyed
before pipe_context.

Firefox with WebGL2 enabled hits this bug.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99456

v2: protect against a double destroy in st_create_context_priv and callers.

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>