mesa: don't lock hashtables that are not shared across contexts

mesa: don't lock hashtables that are not shared across contexts

From Chapter 5 'Shared Objects and Multiple Contexts' of
the OpenGL 4.5 spec:

   "Objects which contain references to other objects include
   framebuffer, program pipeline, query, transform feedback,
   and vertex array objects.   Such objects are called container
   objects and are not shared"

For we leave locking in place for framebuffer objects because
the EXT fbo extension allowed sharing.

We could maybe just replace the hash with an ordinary hash table
but for now this should remove most of the unnecessary locking.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>