drm/nouveau/falcon: use NXTCTX register instead of NEW_INSTBLK

Both registers allow to bind a new context, but NXTCTX will work on all
falcons, while legacy NEW_INSTBLK is reserved to PMU.

After setting NXTCTX we trigger a context switch by writing 0x090 and
0x0a4.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>