drm/ast: AST2000 cannot be detected correctly

commit 83502a5d34386f7c6973bc70e1c423f55f5a2e3a upstream.

Type error and cause AST2000 cannot be detected correctly

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Reviewed-by: Egbert Eich <eich@suse.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>