drm/mgag200: Move CRTC atomic_enable to model-specific code

drm/mgag200: Move CRTC atomic_enable to model-specific code

The CRTC atomic_enable helper contains per-model branches for
G200ER, G200EV and G200SE devices. Implement a dedicated helper
for each of them and remove the branches from the shared helper.

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/20220728124103.30159-14-tzimmermann@suse.de