drm/mgag200: Rename G200WB prepare/commit function

The prepare and commit helpers for G200WB devices control the BMC.
Rename them accordingly. While at it, also change the passed value's
type to struct mga_device and remove some type upcasting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-7-tzimmermann@suse.de