drm/nouveau: fix nv40 pcie gart size

Nouveau sets the PCIE GART size to 64MiB for all cards before nv50,
but nv40 has enough RAMIN space to support 512MiB GART size. This
patch fixes this value to make use of this hardware capability.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>