media: vivid: fix FB dependency

It's not enough to have a dependency on CONFIG_FB, as that can be in
a loadable module when vivid itself is builtin:

drivers/media/test-drivers/vivid/vivid-osd.o: in function `vivid_fb_init':
vivid-osd.c:(.text+0xdc0): undefined reference to `fb_alloc_cmap'
vivid-osd.c:(.text+0xe26): undefined reference to `register_framebuffer'

Change the dependency to only allow configurations that can be built,
but change the FB to FB_CORE so this is also possible when using
DRM with FB compatibility rather than full fbdev.

Fixes: 20889dde ("media: vivid: Introduce VIDEO_VIVID_OSD")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>