fbdev/core: Remove empty internal helpers from fb_logo.c

Remove the two empty helpers for the case the CONFIG_FB_LOGO_EXTRA
has not been set. They are internal functions and only called once.
Providing empty replacements seems like overkill. Instead protect
the call sites with a test for CONFIG_FB_LOGO_EXTRA.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907085408.9354-7-tzimmermann@suse.de

fbdev/core: Move logo functions into separate source file

Move the fbdev function for displaying boot-up logos into their
own file fb_logo.c. Only build fb_logo.c if CONFIG_LOGO has been
selected. No functional changes.

v2:
* include fb_internal.h (kernel test robot)
* simplify option-parsing ifdefs
* build fb_logo.o iff CONFIG_LOGO has been set

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230907085408.9354-6-tzimmermann@suse.de