Very strictly speaking this is possible if you have special hw and
genlocked CRTCs. In general switching a plane between two active CRTC
just won't work so well and is probably not tested at all. Just forbid
it.

I've put this into the core since right now no helper or driver copes
with it, no userspace has code for it and no one asks for it. Yes
there's piles of corner-cases where this would be possible to do this
like:
- switch from inactive crtc to active crtc
- switch from active crtc to inactive crtc
- genlocked display
- invisible plane (to do whatever)
- idle plane hw due to dsi cmd mode/psr
- whatever
but looking at details it's not that easy to implement this correctly.
Hence just put it into the core and add a comment, since the only
userspace we have right now for atomic (weston) doesn't want to use
direct plane switching either.

v2: don't bother with complexity and just outright disallow plane
switching without the intermediate OFF state. Simplifies drivers, we
don't have any hw that could do it anyway and current atomic userspace
(weston) works like this already anyway.

v3: Bikeshed function name (Ville) and add comment (Rob).

v4: Also bikeshed commit message (Rob).

v5: Fix compile warnings reported by 0-day.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Stone <daniels@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>