drm/i915/icl: Drop spurious register read from icl_dbuf_slices_update

drm/i915/icl: Drop spurious register read from icl_dbuf_slices_update

Register DBUF_CTL_S2 is read and it's value is not used. As
there is no explanation why we should prime the hardware with
read, remove it as spurious.

Fixes: 1a8d506ad21f ("drm/i915/icl: Enable 2nd DBuf slice only when needed")
Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109140924.2663-1-mika.kuoppala@linux.intel.com