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: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NBen Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Tested-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>