drm/sysfb: Merge mode-config functions

Provide initializer to set struct drm_mode_config_funcs. Convert
ofdrm and simpledrm.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-8-tzimmermann@suse.de