fbdev: sh_mobile_meram: Remove unneeded sanity checks

The meram_register(), meram_unregister() and meram_update() operations
check that the pointers they get from the caller are not NULL. Those
checks can be remove, as the caller already ensures that the pointers
are valid.

The platform sanity checks can also be removed, as the operations can't
be accessed without valid platform data anyway.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>