drm/nouveau/nvkm: mark expected switch fall-throughs

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

This patch aims to suppress 29 missing-break-in-switch false positives.

Addresses-Coverity-ID: 1456891 ("Missing break in switch")
Addresses-Coverity-ID: 1324063 ("Missing break in switch")
Addresses-Coverity-ID: 1324063 ("Missing break in switch")
Addresses-Coverity-ID: 141432 ("Missing break in switch")
Addresses-Coverity-ID: 141433 ("Missing break in switch")
Addresses-Coverity-ID: 141434 ("Missing break in switch")
Addresses-Coverity-ID: 141435 ("Missing break in switch")
Addresses-Coverity-ID: 141436 ("Missing break in switch")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>