Revert "i965: Disable guardband clipping in the smaller-than-viewport case."

Revert "i965: Disable guardband clipping in the smaller-than-viewport case."

This reverts commit 0bac2551e40410e2251daf4fd9faf69310ab34ce.

Now that we position the guardband correctly (applying translations
in addition to scaling) and made it as large (or larger) than the
render target, this shouldn't be necessary.

Now we leave guardband clipping enabled 100% of the time, like the
Windows driver does.

Fixes GL45-CTS.gtf21.GL2FixedTests.clip.clip.  It tries to draw a
16384x64 rectangle, and it appears that some kind of numerical
imprecisions in the clipper result in some edge pixels going missing.
The Windows driver passes this test because of guardband clipping.

Cc: "17.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>