no more indexes - clean up API

[Dave Airlie]

The screen/scrninfo APIs are very into passing index values, that
are then immediately used to lookup a global array, this is not
an API, its whatever the opposite is.

This set of patches changes all the API passing of screen indices
(apart from the logging functions). It seriously breaks the driver
API/ABI, and I've got a demo of some compat for it I've sent out before.

I'll try and co-ordinate with Keith, Alex/Chris/Ben so that main 3
drivers are compatible when we push the server with all these in it.

I was holding back on this to try and get some bit by bit review, but
it might be easier for someone to just block off an hour and go mad,
so here goes.

Dave.