[efficeon-agp] Add missing memory mask

Original patch by Benjamin Herrenschmidt after debugging by Brian Hinz.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Brian Hinz <bphinz@hotmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>