drm/ast: Remove struct ast_{vga,sil165}_connector

Both, struct ast_vga_connector and struct ast_sil164_connector, are
now wrappers around struct drm_connector. Remove them.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-4-tzimmermann@suse.de