In cases where the scanout hw is sufficiently similar between "overlay"
and traditional crtc layers, it might be convenient to allow the driver
to create internal drm_plane helper objects used by the drm_crtc
implementation, rather than duplicate code between the plane and crtc.
A private plane is not exposed to userspace.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>