anv: Stop generating weak references for instance entrypoints

anv: Stop generating weak references for instance entrypoints

We don't need weak references to instance entrypoints because we never
have more than one of each so we don't need the NULL fall-back.  This
also helps us avoid forgetting things because we now get link errors for
missing instance entrypoints.

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