drm/simpledrm: Use fbdev defaults for shadow buffering

Don't select shadow buffering for the fbdev console explicitly. The
fbdev emulation's heuristic will enable it for any framebuffer with
.dirty callback.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220303205839.28484-2-tzimmermann@suse.de