drm/atomic: Wait for vblank whenever a plane is added to state.

There's 2 reasons for doing a vblank wait:
- To fulfill uabi expectations, but the legacy ioctls are ill-defined
  enough that we really only need this when we do send out an event.
- To make sure we don't tear down mappings before the scanout engine
  stops accessing it.

The later is problematic with the current code since e.g. rotation
might need a different mapping than normal orientation. And rotation
is a plane property, and not on the fb. Hence we need to remove this
optimization.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[danvet: Completely new commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481204729-9058-5-git-send-email-maarten.lankhorst@linux.intel.com