nv50: correctly calculate the number of vertical blocks during transfer map

Cc: "10.0 10.1" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
(cherry picked from commit 882070cc81588a133d81e3ef8790d7375f494eff)