drm/nouveau/gr/gf100: wait on bottom half of FE's pipeline

When emitting the ICMD bundle, wait on the bottom half (bit 3 of the
GR_STATUS register) instead of upper half (bit 2) to make sure methods
are effectively emitted.

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