drm/nv40: attempt to reserve just enough vram for all 32 channels

This also makes the fact we're giving 512MiB of GART space to all PCIE
boards explicit, although the vast majority (if not all) of them will
now have a ramin_rsvd_vram larger than 2MiB anyway.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>