drm/nouveau/bsp/g92: disable by default

G92's seem to require some additional bit of initialization before the
BSP engine can work. It feels like clocks are not set up for the
underlying VLD engine, which means that all commands submitted to the
xtensa chip end up hanging. VP seems to work fine though.

This still allows people to force-enable the bsp engine if they want to
play around with it, but makes it harder for the card to hang by
default.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org