fbdev/au1200fb: Set FBINFO_VIRTFB flag

The au1200fb driver operates on DMA-able 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>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-17-tzimmermann@suse.de