fbdev: Move core fbdev symbols to a separate Kconfig file

The drivers/video/fbdev/Kconfig defines both symbols for fbdev drivers and
core fbdev symbols, that can be enabled independently of the fbdev drivers.

Split the Kconfig in two, one that only has the symbols for fbdev drivers
and another one that contains the fbdev core symbols.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230719081544.741051-3-javierm@redhat.com