drm/i915/skl: Fix WaDisableChickenBitTSGBarrierAckForFFSliceCS

Robert noticed that the FF_SLICE_CS_CHICKEN2 offset was wrong. Ooops.

Ville noticed that the write was wrong since FF_SLICE_CS_CHICKEN2 is a
masked register. Re-oops.

A wonder if went through 2 people while having roughly a bug per line...

The problem was introduced in the original patch:

  commit 2caa3b26
  Author: Damien Lespiau <damien.lespiau@intel.com>
  Date:   Mon Feb 9 19:33:20 2015 +0000

      drm/i915/skl: Implement WaDisableChickenBitTSGBarrierAckForFFSliceCS

v2: Also fix the register write (Ville)

Reported-by: Robert Beckett <robert.beckett@intel.com>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Robert Beckett <robert.beckett@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Nick Hoath <nicholas.hoath@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>