drm/vboxvideo: Switch to generic fbdev emulation

There's nothing special about vboxvideo's fbdev emulation that is
not provided by the generic implementation. Switch over and remove
the driver's code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011134808.3955-2-tzimmermann@suse.de