drm/ast: open key before detect chips

Some config settings like 3rd TX chips will not get correctly
if the extended reg is protected

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Reviewed-by: Egbert Eich <eich@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>