drm/i915:Add the VCS2 switch in Intel_ring_setup_status_page

The Gen7 doesn't have the second BSD ring. But it will complain the switch check
warning message during compilation. So just add it to remove the
switch check warning.

V1->V2: Follow Daniel's comment to update the comment

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>