i915: Gut all remaining bits of hardware fog

i915: Gut all remaining bits of hardware fog

None of this ever gets used.  Fog is always calculated by a fragment
program.  Even though the fixed-function fog unit is never used, state
updates are still sent to the hardware.  Removing those spurious state
updates can't hurt performance.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Corbin Simpson <MostAwesomeDude@gmail.com>
Acked-by: Alex Deucher <alexdeucher@gmail.com>