i965: Use a better guardband calculation.

(Patch co-authored by Jason and Ken.)

We scaled the guardband based on the viewport size, but failed to
take into account the translation portion of the viewport transform.

This meant the guardband was always centered around the origin.
We want it to be centered around the screen-space drawing area,
which is the intersection of the viewport and the render target.

At best, getting this wrong would reduce the guardband's effectiveness
in some cases.  At worst, it might break things - objects outside of the
guardband are trivially rejected, so getting the guardband in the wrong
place and leaving guardband clipping enabled could cause problems.

v2: drop clamping of positive maximums.

Cc: "17.0" <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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