Drop __DRInativeDisplay and pass in __DRIscreen pointers instead.

Drop __DRInativeDisplay and pass in __DRIscreen pointers instead.

Many DRI entry points took a __DRInativeDisplay pointer and a screen
index as arguments.  The only use for the native display pointer was to
pass it back to the loader when looking up the __DRIscreen for the given
screen index.

Instead, let's just pass in the __DRIscreen pointer directly, which
let's drop the __DRInativeDisplay type and the getScreen function.

The assumption is now that the loader will be able to retrieve context
from the __DRIscreen pointer when necessary.