fbdev/au1200fb: Initialize fb_ops with fbdev macros

Initialize the instance of struct fb_ops with fbdev initializer
macros for framebuffers in DMA-able virtual address space. Set the
read/write, draw and mmap callbacks to the correct implementation
and avoid implicit defaults. Also select the necessary helpers in
Kconfig.

Fbdev drivers sometimes rely on the callbacks being NULL for a
default I/O-memory-based implementation to be invoked; hence
requiring 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.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231127131655.4020-18-tzimmermann@suse.de