Remove ScrnInfoRec::privates

The fewer instances of the old "integer index into an array" privates system we retain, the better. xf86AllocateScrnInfoPrivateIndex now has only in-server consumers, and all of them could use privates hanging off the ScreenRec instead.

This series is a bit draconian in that it deletes the old ramdac code. It might in principle make sense to keep ramdac drivers as common code, but in practice all it's buying you is hardware cursor support on some very very very old cards, in particular the s3 tga and glint drivers. glint should probably be fixed up to at least build (which isn't hard), but the other two are basically dead code.