fbcon: call fbcon_fb_(un)registered directly

fbcon: call fbcon_fb_(un)registered directly

With

commit 6104c37094e729f3d4ce65797002112735d49cd1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Aug 1 17:32:07 2017 +0200

    fbcon: Make fbcon a built-time depency for fbdev

we have a static dependency between fbcon and fbdev, and we can
replace the indirection through the notifier chain with a function
call.

v2: Sam Ravnborg noticed that mach-pxa/am200epd.c has a notifier too,
and listens to this.

...

Looking at the code it seems to wait for some fb to show up, so that
it can get the framebuffer base address from the fb_info struct. I
suspect his is some firmware fbdev. Then it uses that information to
let the real fbdev driver (metronomefb.c by the looks) get at the
framebuffer memory.

This doesn't looke like it's easy to fix (except by deleting the
entire thing, seems untouched since 2008, we might be able to get away
with that), so let's just stuff a few #ifdef into fb.h and fbmem.c and
cry over them for a bit.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Yisheng Xie <ysxie@foxmail.com>
Cc: Peter Rosin <peda@axentia.se>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: linux-fbdev@vger.kernel.org
Cc: Daniel Mack <daniel@zonque.org>
Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Konstantin Khorenko <khorenko@virtuozzo.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Steve Sakoman <sakoman@gmail.com>
Cc: Steve Sakoman <steve@sakoman.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190528090304.9388-11-daniel.vetter@ffwll.ch