drm/nouveau/gr/nv3x: fix instobj write offsets in gr setup

This should fix some unaligned access warnings. This is also likely to
fix non-descript issues on nv30/nv34 as a result of incorrect channel
setup.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96836
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>