drm/i915: Get VECS semaphore info on error

Ideally we could use for_each_ring with the ring flags as I've done a
couple times
(http://lists.freedesktop.org/archives/intel-gfx/2013-June/029450.html

).
Until Daniel merges that patch though, we can just use this.

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>