drm: Move sysfb drivers into separate subdirectory

The ofdrm and simpledrm drivers are special as they operate on
externally provided framebuffers. Move them into their own sub-
directory. Will let them share common code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250401094056.32904-5-tzimmermann@suse.de