video: export fb_prepare_logo

Some drivers that may be loadable modules use the fb_prepare_logo
function, so we have to export it. Found during randconfig
builds with mmpfb.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>