drm/ast: Rename struct ast_connector to struct ast_vga_connector

Prepare for introducing other connectors besides VGA. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-6-tzimmermann@suse.de