drm/ast: Zero is missing in detect function

The function ast_get_modes() will also return 0, when it try to get the
edid, but it also do not get the edid.

Signed-off-by: Ainux.Wang <ainux.wang@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716015615.9150-1-ainux.wang@gmail.com