drm/atomic-helpers: Recover full cursor plane behaviour

Cursor plane updates have historically been fully async and mutliple
updates batched together for the next vsync. And userspace relies upon
that. Since implementing a full queue of async atomic updates is a bit
of work lets just recover the cursor specific behaviour with a hint
flag and some hacks to drop the vblank wait.

v2: Fix kerneldoc, reported by Wu Fengguang.

Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>