video/cmdline: Hide __video_get_options() behind CONFIG_FB_CORE

The function __video_get_options() only exists for compatibility
with old fbdev drivers that cannot be refactored easily. Hide it
behind CONFIG_FB_CORE.

v2:
	* support CONFIG_FB_CORE=m via IS_ENABLED() (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240118090721.7995-3-tzimmermann@suse.de