drm/fb-helper: Support smem_len in deferred I/O

The size of the framebuffer can either be stored in screen_info or
smem_len. Take both into account in the deferred I/O code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230320150751.20399-5-tzimmermann@suse.de