agpgart: detected ALi M???? chipset with M1621

Add M1621 chipset name to ali-agp, preventing "Detected ALi M???? chipset"
message.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>