drm/i915: i915_gem_object_sync must handle NULL

When I extracted the synchronization code for implementing semaphorified
pageflips (74f5f6e0), I neglected the non pipelined case which also
calls this code. The modesetting code wants to make sure the object has
finished rendering to the frame before configuring the scanout (ie.
non-pipelined case).

As a result of a follow on discussion on IRC, I've decided to add a
comment about the function itself which received much inspiration from
Chris as well. So really, this patch was ghost-written by Chris :).

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>