dri: Reference the global driver vtable once at screen init..

dri: Reference the global driver vtable once at screen init..

This is part of the prep for megadrivers, which won't allow using a single
global symbol due to the fact that there will be multiple drivers built
into the same dri.so file.  For that, we'll need screen init to take a
reference to the driver to set up this vtable.

v2: Fix two missed references to driDriverAPI.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org> (v1)