drm/sysfb: Merge connector functions

Merge the connector functions of ofdrm and simpledrm. Replace the
code in each driver with the shared helpers. Set up callbacks with
initializer macros.

No effective code changes. The sysfb connector only returns the
preconfigured display mode.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-9-tzimmermann@suse.de