drm/mgag200: Split mgag200_modeset_init()

Split mgag200_modeset_init() into smaller helpers to initialize
the mode_config structure and the pipeline. This will be helpful
for transforming this code into per-model functions. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220728124103.30159-2-tzimmermann@suse.de