drm/ast: Add detect function support

The existence of the connector cannot be detected,
so add the detect function to support.

Signed-off-by: Ainux <ainux.wang@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210526111515.40015-1-ainux.wang@gmail.com