drm/ast: Use drm_connector_helper_get_modes()

The .get_modes() code for VGA and SIL164 connectors does not depend
on either type of connector. Replace the driver code with the common
helper drm_connector_helper_get_modes(). It reads EDID data via
DDC and updates the connector's EDID property.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-12-tzimmermann@suse.de