fbdev: export symbol fb_mode_option

Frame buffer and mode setting drivers can be built as modules,
so fb_mode_option needs to be exported to support these.

Prevents this error:

  ERROR: "fb_mode_option" [drivers/ps3/ps3av_mod.ko] undefined!

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>