agp/uninorth: Allow larger aperture sizes on pre-U3 bridges.

Using the radeon KMS test functionality, I verified that the AGP bridge of the
Intrepid2 chipset in my PowerBook supports aperture sizes up to 256M. So allow
aperture sizes up to 256M on pre-U3 bridges as well, and bump the default size
to 256M. It's possible that older revisions only support smaller sizes, but
it'll be easy to verify that with the raden KMS test functionality. Also,
there's only a problem on an actual attempt to access the aperture beyond the
maximum size supported by the hardware, and non-KMS X still defaults to using
only 32M.

Also use ARRAY_SIZE for the aperture size arrays.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>