drm/i915: warn if ring is active before sync flush

Based on Bspec the command parser must be stopped prior to
issuing sync flush. This should be done by the caller of
intel_ring_setup_status_page. Patch adds a warning if it is
not done.

v2: rebased based on new patch (wait for ring to become idle)

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>