drm/mgag200: Move ER/EW3 register initialization to per-model code

The register initialization code contains special cases for G200ER
and G200EW3 hardware. Move this to per-model code.

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/20220728124103.30159-4-tzimmermann@suse.de