[AGP] Fix intel 965 AGP memory mapping function

This introduces a i965-specific "mask_memory()" function that knows
about the extended physical addresses that the i965 supports.  This
allows us to correctly map in physical memory in the >4GB range into the
GTT.

Also simplify/clean-up the i965 case for the aperture sizing by just
returning the fixed 512kB size from "fetch_size()".  We don't really
care that not all of the aperture may be visible - the only thing that
cares about the aperture size is the Intel "stolen memory" calculation,
which depends on the fixed size.

Cc: Keith Packard <keithp@keithp.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>