vkencoder-private: clean ups and fixes
- vkencoder-private: remove duplicated structure definition
- vkencoder-private: don't override error on get_format() call
- vkencoder-private: There's no need to store the aligned offset of 0
- vkencoder-private: use g_clear_pointer to unref packed headers
- vkencoder-private: don't check twice for encoder parameter
- vkencoder-private: fix code style