drm/i915/gt: Disable arbitration around Braswell's pdp updates

Braswell's pdp workaround is full of dragons, that may be being angered
when they are interrupted. Let's not take that risk and disable
arbitration during the update.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111105735.21515-1-chris@chris-wilson.co.uk