drm/plane-helper: Adapt cursor hack to transitional helpers

In

commit f02ad907
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jan 22 16:36:23 2015 +0100

    drm/atomic-helpers: Recover full cursor plane behaviour

we've added a hack to atomic helpers to never to vblank waits for
cursor updates through the legacy apis since that's what X expects.
Unfortunately we've (again) forgotten to adjust the transitional
helpers. Do this now.

This fixes regressions for drivers only partially converted over to
atomic (like i915).

Reported-by: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>