i965: Disable unlit-centroid workaround on Gen < 6.

i965: Disable unlit-centroid workaround on Gen < 6.

Back to the original commit (8313f444) adding the workaround, we were
enabling it on gens <= 7, even though gens <= 5 can't do multisampling.

I cannot find documentation that says that Sandybridge needs this
workaround but in practice disabling it causes these piglit tests to
fail:

EXT_framebuffer_multisample/interpolation {2,4} centroid-deriv{,-disabled}

On Ironlake:

total instructions in shared programs: 4358478 -> 4349671 (-0.20%)
instructions in affected programs:     117680 -> 108873 (-7.48%)

A bunch of shaders in TF2, Portal 2, and L4D2 are cut by 25~30%.

Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>