drm/cirrus: Integrate connector into pipeline code

Integrate the connector with the rest of the pipeline setup code.
Move some helpers within the file and adapt naming slightly. No
functional changes.

Done in preparation of converting cirrus to regular atomic helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-7-tzimmermann@suse.de