nv30: go back to not using viewport validate function for swtnl

nv30: go back to not using viewport validate function for swtnl

The output of draw requires a null viewport transform, which the regular
code is ill-equiped to do. Reinstate the original settings in the render
path, and add setting of the viewport clip polygon based on fb
width/height (as that is all taken care of by draw).

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>