freedreno: Take a lock around shader variant creation.

freedreno: Take a lock around shader variant creation.

Shaders are shared across contexts in gallium (part of making it so
that you get shader compile at link time, for shader-db and to reduce
compiles at draw time).  So, we need to protect from variant creation
for a shader from multiple threads at the same time.

Reviewed-by: Rob Clark <robdclark@gmail.com>