fbdev: Harmonize some comments in <linux/fb.h>

Make the comments for I/O, system and DMA memory say the same.
Makes the header file's structure more obvious.

Suggested-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707083422.18691-13-tzimmermann@suse.de