fbcon: Make fbcon_registered_fb and fbcon_num_registered_fb static

fbcon_registered_fb and fbcon_num_registered_fb is not referred outside
drivers/video/fbdev/core/fbcon.c, so make them static.

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230803020939.491-1-guozihua@huawei.com