nv50,nvc0: update sampler/view bind functions to accept NULL array

Apparently vl (or vdpau) wants to pass that in now. Handle it.

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 c52b057e)