atyfb: mark DMI system id table as __initconst

We can mark the DMI system id table as __initconst by using a helper
variable that'll tell us if we need to unregister the reboot notifier in
atyfb_exit() instead of matching the DMI system id again.

This frees up ~680 bytes of runtime memory, the DMI table occupies.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>