drm/mgag200: Replace module-init boiler-plate code with DRM helpers

Remove custom mgag200_init() and mgag200_exit() functions and initialize
the module with DRM_module helpers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217144615.32733-7-tzimmermann@suse.de