drm/i915: VCS is not the last ring

I made a mistake, please forgive me.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=48254


Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>