alpha: AGP update (fixes compile failure)

This brings Alpha AGP platforms in sync with the change to struct
agp_memory (unsigned long *memory => struct page **pages).

Only compile tested (I don't have titan/marvel hardware), but this change
looks pretty straightforward, so hopefully it's ok.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Dave Airlie <airlied@linux.ie>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>