Update serverabi generation to also account for driver Provides

* Have the drivers use the file serverminver instead of serverabiver in order
  to generate the versioned dependency on the server. This is clearer
  naming.
* Use serverabiver to actually store the server's video ABI version. This
  is used to generate the Provides: line for drivers so the server can
  depend on them properly. This goes in to the substvar
  xviddriver:Provides.