It would race between userspace thread and commit worker.  Ie. vblank
irq would trigger event and userspace could begin the next atomic
update, before the commit worker had a chance to clear the pending
flag.

If we do end up needing something to prevent userspace from trying
another pageflip before getting vblank event, it should probably be
implemented as a pending_planes bitmask, similar to pending_crtcs.  See
start_atomic() and end_atomic().
Signed-off-by: Rob Clark <robdclark@gmail.com>