drm/ofdrm: Open-code drm_simple_encoder_init()

The helper drm_simple_encoder_init() is a trivial helper around
drm_encoder_init() and therefore deprecated. Open-code the function
and remove the dependency.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-3-tzimmermann@suse.de