i965: Disable the GB clip test when a limited viewport is set.

The theory of the guardband is that you extend the clip volume to avoid
expensive clipping computation, and just let fragments outside the viewport
get clipped by the drawable's bounds.  But if a smaller-than-window-size
viewport is set, and we don't also happen to have a scissor set, then
rendering could incorrectly extend outside of the viewport when it should have
been clipped to the viewport.

Fixes the new piglit triangle-guardband-viewport test.

Reviewed-by: Kenneth Graunke <[email protected]>
NOTE: This is a candidate for the 9.0 branch.

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