Now that all drivers are converted over, we can make a few changes.
First off, vk_device_init no longer takes two separate allocators
because we can assume that the parent instance is non-null and it can
pull the instance allocator from that.  Second, dispatch tables and the
instance extension table are no longer optional.  We leave the device
extension table optional for now because we don't do any verification at
vk_init_physical_device time and some drivers find it more convenient to
set the extensions later in their own physical_device_init for various
reasons.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <mesa/mesa!8676>