drm/nouveau/gr/gf100-: stop using NV_PGRAPH_FECS_CTXSW_MAILBOX_CLEAR

This doesn't work on Ampere for some reason, switch to directly modifying
NV_PGRAPH_FECS_CTXSW_MAILBOX instead.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>