i965: Be more clever about setting up our viewport clip

i965: Be more clever about setting up our viewport clip

Before, we were trusting in the hardware to take the intersection
of the viewport clip with the drawing rectangle.  Unfortunately,
3DSTATE_DRAWING_RECTANGLE is fairly expensive because it implicitly
does a full pipeline stall.  If we're a bit more careful with our
viewport clipping, we can just re-emit it once at context creation
time.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>