vulkan,anv: Add a common base object type for VkDevice

vulkan,anv: Add a common base object type for VkDevice

We should keep this very minimal; I don't know that we need to go all
struct gl_context on it.  However, this gives us at least a tiny base on
which we can start building some common functionality.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4690>