anv: Add a concept of a descriptor buffer

anv: Add a concept of a descriptor buffer

This buffer goes along side the CPU data structure and may contain
pointers, bindless handles, or any other descriptor information.
Currently, all descriptors are size zero and nothing goes in the buffer
but this commit sets up the framework we will need later.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>