drm/nv10: Change the BO size threshold determining the memory placement range.

Fixes the framebuffer memory allocation failure seen on some
low-memory cards, followed by X refusing to start.

https://bugs.freedesktop.org/show_bug.cgi?id=42384

Reported-by: Chris Paulson-Ellis <chris@edesix.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>