i965/msaa: Fix centroid interpolation of unlit pixels.

From the Ivy Bridge PRM, Vol 2 Part 1 p280-281 (3DSTATE_WM:
Barycentric Interpolation Mode):

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

To work around this problem, after doing centroid interpolation, we
replace the centroid-interpolated values for unlit pixels with
non-centroid-interpolated values (which are interpolated at pixel
centers).  This produces correct rendering at the expense of a slight
increase in shader execution time.

I've conditioned the workaround with a runtime flag
(brw->needs_unlit_centroid_workaround) in the hopes that we won't need
it in future chip generations.

Fixes piglit tests "EXT_framebuffer_multisample/interpolation {2,4}
{centroid-deriv,centroid-deriv-disabled}".  All MSAA interpolation
tests pass now.

Reviewed-by: Eric Anholt <[email protected]>

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