video: convert mbxfb to use module_platform_driver()

This patch converts mbxfb to use the module_platform_driver() macro
which makes the code smaller and a bit simpler.

Cc: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>