fbdev/core: Unexport logo helpers

The interfaces for the fbdev logo are not used outside of the fbdev
module. Hence declare the fbdev logo functions in the internal header
file and remove their symbol exports. Only build the functions if
CONFIG_LOGO has been selected.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907085408.9354-5-tzimmermann@suse.de