i965: Silence compiler warning about promoted_constants.

We only have a cfg != NULL if we went through one of the paths that set
it, but my compiler doesn't figure that out.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 6411defdcd ("intel/cs: Re-run final NIR optimizations for each SIMD size")

src/intel/compiler/brw_fs.cpp

@@ -7223,7 +7223,7 @@ brw_compile_cs(const struct brw_compiler *compiler, void *log_data,
    fs_visitor *v8 = NULL, *v16 = NULL, *v32 = NULL;
    cfg_t *cfg = NULL;
    const char *fail_msg = NULL;
-   unsigned promoted_constants;
+   unsigned promoted_constants = 0;
 
    /* Now the main event: Visit the shader IR and generate our CS IR for it.
     */