drm/mgag200: Integrate init function into load function

Done to simplify initialization code before embedding the DRM device
instance in struct mga_device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507090315.21274-4-tzimmermann@suse.de