i965: Initialize all member variables of cfg_t on construction.

The cfg_t object relies on the memory allocator zeroing out its
contents before it's initialized, which is quite an unusual practice
in the C++ world because it ties objects to some specific allocation
scheme, and gives unpredictable results when an object is created with
a different allocator -- Stack allocation, array allocation, or
aggregation inside a different object are some of the useful
possibilities that come to my mind.  Initialize all fields from the
constructor and stop using the zeroing allocator.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_cfg.cpp

@@ -82,6 +82,7 @@ cfg_t::create(void *parent_mem_ctx, exec_list *instructions)
 {
    mem_ctx = ralloc_context(parent_mem_ctx);
    block_list.make_empty();
+   blocks = NULL;
    num_blocks = 0;
    ip = 0;
    cur = NULL;

src/mesa/drivers/dri/i965/brw_cfg.h

@@ -60,7 +60,7 @@ public:
 
 class cfg_t {
 public:
-   DECLARE_RZALLOC_CXX_OPERATORS(cfg_t)
+   DECLARE_RALLOC_CXX_OPERATORS(cfg_t)
 
    cfg_t(backend_visitor *v);
    cfg_t(void *mem_ctx, exec_list *instructions);