drm/mgag200: Consolidate VGA output

The various models have common code for the VGA output's encoder and
connector. Move everything into a single shared source file. Remove some
obsolete initializer macros. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610141141.29527-2-tzimmermann@suse.de