drm/cirrus: Convert to regular atomic helpers

Replace simple-KMS helpers with DRM's regular helpers for atomic
modesetting. Avoids the mid-layer and the additional wrappers around
GEM's shadow-plane helpers.

Most of the simple-KMS code is just wrappers around regular atomic
helpers. The conversion is therefore equivalent to pulling the
simple-KMS helpers into cirrus and removing all the intermediate
code and data structures between the driver and the atomic helpers.
As the simple-KMS helpers lump primary plan, CRTC and encoder into a
single data structure, the conversion to regular helpers allows to
split modesetting from plane updates and handle each individually.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-9-tzimmermann@suse.de