v3dv: move error string definition to debug path

No point in computing this if debug is disabled.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7848>

src/broadcom/vulkan/v3dv_util.c

@@ -45,12 +45,12 @@ __vk_errorf(struct v3dv_instance *instance, VkResult error, const char *file,
    va_list ap;
    char buffer[256];
 
-   const char *error_str = vk_Result_to_str(error);
-
 #ifndef DEBUG
       return error;
 #endif
 
+   const char *error_str = vk_Result_to_str(error);
+
    if (format) {
       va_start(ap, format);
       vsnprintf(buffer, sizeof(buffer), format, ap);