fbdev/udlfb: Select correct helpers

The driver uses deferred I/O. Select the correct helpers via
FB_SYSMEM_HELPERS_DEFERRED in the Kconfig file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-16-tzimmermann@suse.de