video: clarify I2C dependencies

We can only use I2C support in frame buffer drivers if
Either I2C is built-in, or both I2C and the driver itself
are loadable modules.

Fix this dependency for MB862XX and CyberPro frame buffers.

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
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>