Retain drm_plane tracking objects for all actual DRM planes when using
universal planes, not just overlay planes. Rename uses of 'sprite' to
'plane' to make it clear that it can now be any kind of plane, not just
an overlay/sprite.

These are currently unused.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>