drm/mgag200: Initialize each model in separate function

Add a separate initializer function for each model. Add separate
devic structures for G200 and G200SE, which require additional
information.

Also move G200's and G200SE's helpers for reading the BIOS and
version id into model-specific code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112522.5774-3-tzimmermann@suse.de