i965/fs: Improve performance of varying-index uniform loads on IVB.

Like we have done for the VS and for constant-index uniform loads, we use
the sampler engine to get caching in front of the L3 to avoid tickling the
IVB L3 bug.  This is also a bit of a functional change, as we're now
loading a vec4 instead of a single dword, though we're not taking
advantage of the other 3 components of the vec4 (yet).

With the driver hacked to always take the varying-index path for all
uniforms, improves performance of my old GLSL demo by 315% +/- 2% (n=4).
This a major fix for some blur shaders in compositors from the
varying-index uniforms support I introduced in 9.1.

v2: Move old offset computation into the pre-gen7 path.

Reviewed-by: Kenneth Graunke <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61554
NOTE: This is a candidate for the 9.1 branch.

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