drm/ast: Move SIL164-based connector code into separate helpers

Add helpers for initializing SIL164-based connectors. These used to be
handled by the VGA connector code. But SIL164 provides output via DVI-I,
so set the encoder and connector types accordingly.

If a SIL164 chip has been detected, ast will now create a DVI-I
connector instead of a VGA connector.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-10-tzimmermann@suse.de