nouveau: rewrite winsys in terms of drm_api, support dri2 state tracker

nouveau: rewrite winsys in terms of drm_api, support dri2 state tracker

drm_api is a set of hooks used by the dri2 state tracker, this wraps our
dri1 code around the same set of hooks.

Currently the dri2 build will produce nouveau_dri2.so which you'll need
to install as nouveau_dri.so if you wish to try it.  The dri2 state
tracker doesn't make it easy for a driver to support both paths in the
same binary.