Turned out to be much simpler on top of my latest atomic stuff than
what I've feared. Some details:

- Drop the modeset_lock_all snakeoil in drm_plane_init. Same
  justification as for the equivalent change in drm_crtc_init done in

	commit d0fa1af4
	Author: Daniel Vetter <daniel.vetter@ffwll.ch>
	Date:   Mon Sep 8 09:02:49 2014 +0200

	    drm: Drop modeset locking from crtc init function

  Without these the drm_modeset_lock_init would fall over the exact
  same way.

- Since the atomic core code wraps the locking switching it to
  per-plane locks was a one-line change.

- For the legacy ioctls add a plane argument to the locking helper so
  that we can grab the right plane lock (cursor or primary). Since the
  universal cursor plane might not be there, or someone really crazy
  might forgoe the primary plane even accept NULL.

- Add some locking WARN_ON to the atomic helpers for good paranoid
  measure and to check that it all works out.

Tested on my exynos atomic hackfest with full lockdep checks and ww
backoff injection.

v2: I've forgotten about the load-detect code in i915.

v3: Thierry reported that in latest 3.18-rc vmwgfx doesn't compile any
more due to

commit 21e88620
Author: Rob Clark <robdclark@gmail.com>
Date:   Thu Oct 30 13:39:04 2014 -0400

    drm/vmwgfx: fix lock breakage

Rebased and fix this up.

Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>