drm/nouveau/bios/timing: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1260018
Addresses-Coverity-ID: 1260019
Addresses-Coverity-ID: 1260022
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>