gallium: rename pipe_buffer_handle to pipe_buffer, rework pipebuffer/ code

Provide an actual definition of the pipe_buffer struct,  containing
the parameters used to create the buffer, and its refcount.

Shift refcounting buffers out of the winsys interface, similar to
surfaces & textures.

Rework pipebuffer/ to reflect the fact these changes, and also Michel's
reworking of the buffer interface.

src/mesa/state_tracker/st_atom_constbuf.c

@@ -37,6 +37,7 @@
 #include "pipe/p_context.h"
 #include "pipe/p_defines.h"
 #include "pipe/p_winsys.h"
+#include "pipe/p_inlines.h"
 
 #include "st_context.h"
 #include "st_atom.h"
@@ -70,7 +71,7 @@ void st_upload_constants( struct st_context *st,
       _mesa_load_state_parameters(st->ctx, params);
 
       if (cbuf->buffer && cbuf->size != paramBytes)
-	 ws->buffer_reference( ws, &cbuf->buffer, NULL );
+	 pipe_buffer_reference( ws, &cbuf->buffer, NULL );
 
       if (!cbuf->buffer) {
          cbuf->buffer = ws->buffer_create(ws, 1, PIPE_BUFFER_USAGE_CONSTANT,