drm/ast: Implement atomic enable/disable for encoders

The CRTC helpers contain code to enable and disable DisplayPort
connectors. Implement this functionality in the respective connector's
atomic_enable/atomic_disable callbacks. DRM's atomic-modesetting
helpers will call the functions as part of the atomic commit.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240627153638.8765-2-tzimmermann@suse.de