i965: Disable the unlit centroid workaround on Gen7.

Once upon a time (commit 8313f44409) Paul added code for the unlit
centroid workaround (WaCopyUnlitCentroidBarys). His commit message
claims it fixed the EXT_framebuffer_multisample/interpolation {2,4}
{centroid-deriv,centroid-deriv-disabled} piglit tests but does not say
on which platform, though he cites the IVB PRM.

"3DSTATE_WM [DevIVB, DevHSW]" says

   "[DevIVB]: Workaround: When Centroid Barycentric mode is required, HW
    may produce incorrect interpolation results when a 2X2 pixels have
    unlit pixels."

I later disabled it for Haswell (commit f6db414f3c) with no known ill
effects.

The Sandybridge page does not have this text, but the workarounds
database (see WaCopyUnlitCentroidBarys) says the issues applies *only*
to Sandybridge, and in fact in commit 1a2de7dce8fc I note that disabling
the workaround on Sandybridge causes the tests Paul originally mentioned
to fail.

So this is, and always has been, a huge confusing mess.

Disabling the workaround indeed causes the tests Paul originally
mentioned to fail on Sandybridge but not on Ivybridge/Baytrail.

On Ivybridge:

   total instructions in shared programs: 6914901 -> 6909599 (-0.08%)
   instructions in affected programs: 106766 -> 101464 (-4.97%)
   helped: 884

   total cycles in shared programs: 70874764 -> 70813774 (-0.09%)
   cycles in affected programs: 794144 -> 733154 (-7.68%)
   helped: 688
   HURT: 186

   LOST:   1
   GAINED: 6

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

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