auxdisplay/ht16k33: Set FBINFO_VIRTFB flag

The ht16k33 driver operates on system memory. Mark the framebuffer
accordingly. Helpers operating on the framebuffer memory will test
for the presence of this flag.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Robin van der Gracht <robin@protonic.nl>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-10-tzimmermann@suse.de