hid/picolcd_fb: Set FBINFO_VIRTFB flag

The picolcd_fb 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: "Bruno Prémont" <bonbons@linux-vserver.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Bruno Prémont <bonbons@linux-vserver.org>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-12-tzimmermann@suse.de