Without a lock, two threads may bind the same shader CSO simultaneously,
allocate the same variant simultaneously, and then race each other in
the compiler. This manifests in various ways, most commonly failing the
assertion that UBO pushing has only run once. The simple_mtx_t solution
is used in Iris. Fixes the crash in:

dEQP-EGL.functional.sharing.gles2.multithread.simple.buffers.bufferdata_render

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Cc: mesa-stable