drm/i915: Don't ban default context when stop_rings!=0

If we've explicitly stopped the rings for testing purposes, don't ban
the default context. Fixes kms_flip hang tests.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>