drm/i915: Ignore valid but unknown semaphores

If we find a ring waiting on a semaphore for another assigned but not yet
emitted request, treat it as valid and waiting.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160909131201.16673-18-chris@chris-wilson.co.uk