drm/i915/gt: Leave RING_BB_STATE to default value

Do not reset RING_BB_STATE, leaving it to the default state value. This
prevents bdw/bsw from getting confused when executing batches from the
GGTT.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191230165821.3840449-2-chris@chris-wilson.co.uk