drm/mgag200: Move mode-config to model-specific code

Move the mode-config code into model-specific code and call the
plane/CRTC helpers as needed. This will help with providing per-
model implementations of individual helpers.

Duplication of the pipeline init function is accepted. Some macros
simplify this for shared helpers.

v3:
	* clean up style

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/20220728124103.30159-13-tzimmermann@suse.de