drm/nouveau: fix missing break in switch statement

The NOUVEAU_GETPARAM_PCI_DEVICE case is missing a break statement and falls
through to the following NOUVEAU_GETPARAM_BUS_TYPE case and may end up
re-assigning the getparam->value to an undesired value. Fix this by adding
in the missing break.

Detected by CoverityScan, CID#1460507 ("Missing break in switch")

Fixes: 359088d5 ("drm/nouveau: remove trivial cases of nvxx_device() usage")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>