nvc0: allow a non-user buffer to be bound at position 0

Previously the code only handled it for positions 1 and up (as would be
for UBO's in GL). It's not a lot of trouble to handle this, and vl or
vdpau want this.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111213


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 9f8ed5aa)