drm/fb-helper: Use fb_{cfb,sys}_{read, write}()

Implement DRM fbdev helpers for reading and writing framebuffer
memory with the respective fbdev functions. Removes duplicate
code.

v2:
	* rename fb_cfb_() to fb_io_() (Geert)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Helge Deller <deller@gmx.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230428122452.4856-20-tzimmermann@suse.de