drm/nouveau/bios: memset dcb struct to zero before parsing

commit 595d373f1e9c9ce0fc946457fdb488e8a58972cd upstream.

Fixes type/mask calculation being based on uninitialised data for VGA
outputs.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>