gallium: Unify reference counting.

The core reference counting code is centralized in p_refcnt.h.

This has some consequences related to struct pipe_buffer:

* The screen member of struct pipe_buffer must be initialized, or
  pipe_buffer_reference() will crash trying to destroy a buffer with reference
  count 0. u_simple_screen takes care of this, but I may have missed some of
  the drivers not using it.
* Except for rare exceptions deep in winsys code, buffers must always be
  allocated via pipe_buffer_create() or via screen->*buffer_create() rather
  than via winsys->*buffer_create().

src/mesa/state_tracker/st_atom_constbuf.c

@@ -73,7 +73,7 @@ void st_upload_constants( struct st_context *st,
       /* We always need to get a new buffer, to keep the drivers simple and
        * avoid gratuitous rendering synchronization.
        */
-      pipe_buffer_reference(pipe->screen, &cbuf->buffer, NULL );
+      pipe_buffer_reference(&cbuf->buffer, NULL );
       cbuf->buffer = pipe_buffer_create(pipe->screen, 16, PIPE_BUFFER_USAGE_CONSTANT,
 					paramBytes );