drm/ast: Remove unused value dp501_maxclk

Remove reading the link-rate. The value is maintained by the connector
code but never used.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220207141544.30015-5-tzimmermann@suse.de