drm/mgag200: Use managed device initialization

The mgag200 driver now uses managed functions for DRM devices. The
individual helpers for modesetting and memory managed are already
covered, so only device allocation and initialization is left for
conversion.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605135803.19811-15-tzimmermann@suse.de