radeonsi: support thread-safe shaders shared by multiple contexts

radeonsi: support thread-safe shaders shared by multiple contexts

The "current" shader pointer is moved from the CSO to the context, so that
the CSO is mostly immutable.

The only drawback is that the "current" pointer isn't saved when unbinding
a shader and it must be looked up when the shader is bound again.

This is also a prerequisite for multithreaded shader compilation.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>