video: mbxfb: use dev_get_platdata()

Use the wrapper function for retrieving the platform data instead of
accessing dev->platform_data directly. This is a cosmetic change
to make the code simpler and enhance the readability.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>