i965: Correct gen6+ guardband calculation.

i965: Correct gen6+ guardband calculation.

Too much attention was paid to the first paragraphs, and not enough to
the last little note that "oh, by the way, the rendered things
themselves still have to be clipped to just 8192 wide/high".

Fixes GTF's clip.c test with 4096 or higher width on ivb, where one of
the triangles got the upper half of its pixels dropped.

Tested-by: Ian Romanick <ian.d.romanick@intel.com>