fbdev: Make registered_fb[] private to fbmem.c

No driver access this anymore, except for the olpc dcon fbdev driver but
that has been marked as broken anyways by commit de0952f2

 ("staging:
olpc_dcon: mark driver as broken").

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220725075400.68478-1-javierm@redhat.com