ra: Don't use regs as the ralloc context.

The i965 backends pass something out of 'screen', which is allocated
per-process, making using this as a ralloc context not thread-safe.

All callers ra_alloc_interference_graph() already ralloc_free() its
return value.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

src/util/register_allocate.c

@@ -374,7 +374,7 @@ ra_alloc_interference_graph(struct ra_regs *regs, unsigned int count)
    struct ra_graph *g;
    unsigned int i;
 
-   g = rzalloc(regs, struct ra_graph);
+   g = rzalloc(NULL, struct ra_graph);
    g->regs = regs;
    g->nodes = rzalloc_array(g, struct ra_node, count);
    g->count = count;