drm/fbdev-shmem: Remove obsolete setup function

The old setup function drm_fbdev_shmem_setup() is unused. Remove it
and its internal callbacks. New drivers should call drm_client_setup()
instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-64-tzimmermann@suse.de