Convert all DRI entrypoints to take pointers to __DRI* types.

Convert all DRI entrypoints to take pointers to __DRI* types.

The entrypoints take a mix of __DRIscreen * and void * (screen private)
arguments (similarly for contexts and drawables).  This patch does away
with passing the private void pointer and always only passes the fully
typed __DRIscreen pointer and always as the first argument.

This makes the interface more consistent and increases type safety, and
catches a bug where we would pass a screen private to DRIdrawable::getSBC.