drm/i915: Remove BXT disable pixel mask clamping w/a

This w/a (WaSetDisablePixMaskCammingAndRhwoInCommonSliceChicken) was only
used for preproduction hw, which is no longer in use. Remove the
workaround to simplify the code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123130601.2281-4-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>