drm/i915: Remove CNL from WA 827

CNL A stepping was the only affected there.
But also it is time to clean old pre-production
CNL Workarounds, so let's just remove and clean
this W/A.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181031162845.12419-4-rodrigo.vivi@intel.com