drm/nouveau: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a couple
of warnings by explicitly adding a couple of break statements instead
of letting the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>