nv50,nvc0: guard against zero-size blits

The current state tracker can generate these sometimes. Fixing this is
more involved, and due to some integer math we can generate
divisions-by-zero.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 9bf06141)