drm/i915: Remove BXT restore arbitration around ctx switch

drm/i915: Remove BXT restore arbitration around ctx switch

This w/a (WaDisableCtxRestoreArbitration) 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/20170123130601.2281-3-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>