drm/i915/skl: revert duplicated WaBarrierPerformanceFixDisable:skl

With this simple git diff command one can see that skl_init_workarounds()
got two copies of WaBarrierPerformanceFixDisable:skl:

 git diff -U21 ca6e4405^1 ca6e4405 drivers/gpu/drm/i915/intel_ringbuffer.c

This happened when the backmerge of drm-intel-fixes-2015-07-15
Merged the same fix on both sides. Same fix but not identical enough for
git: with a different surrounding context; hence the code duplication.

This commit merely reverts the output of the git command above
 = the duplication introduced in the backmerge.

(This duplication was found while running git sanity checks on a
_linearized_ i915 forklift for ChromeOS.)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>