Initialize the instance of struct fb_ops with fbdev initializer
macros for framebuffers in I/O address space. This explictily sets
the read/write, draw and mmap callbacks to the correct default
implementation.

Fbdev drivers sometimes rely on the callbacks being NULL for a
default implementation to be invoked; hence requireing the I/O
helpers to be built in any case. Setting all callbacks in all
drivers explicitly will allow to make the I/O helpers optional.
This benefits systems that do not use these functions.

Set the callbacks via macros. No functional changes.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Andy Walls <awalls@md.metrocast.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-media@vger.kernel.org
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-21-tzimmermann@suse.de