drm/i915: disable rings before HW status page setup

Rings should be idle before issuing sync_flush
(in intel_ring_setup_status_page). This patch moves the ring
disabling before doing the HW status page setup.

Signed-off-by: Naresh Kumar Kachhi <naresh.kumar.kachhi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>