drm/nouveau/drm/nouveau/mmu: fix odd_ptr_err.cocci warnings

The kbuild test bot complained about a new coccinelle warning nearby,
which sparked a discussion about the assignment to 'memory' inside of
the conditional expression.  See Link below for the original post.

Fix the assignment to silence the coccinelle warning and also make the
code look a little nicer.

Link: https://lists.freedesktop.org/archives/nouveau/2017-November/029242.html
Signed-off-by: Christoph Böhmwalder <christoph@boehmwalder.at>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>