drm/nv17-nv40: Fix modesetting failure when pitch == 4096px (fdo bug 35901).

Reported-by: Mario Bachmann <grafgrimm77@gmx.de>
Tested-by: Greg Turner <gmturner007@ameritech.net>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>