drm/nouveau/gpio: check function 76 in the power check as well

Added GPIO is "Power Alert". It's uncertain if this
GPIO is set on GPU initialization or only if a change is detected by the
GPU at runtime.

This GPIO can be found on Tesla and sometimes on Fermi GPUs.

Untested, wrote according to documentation.

Signed-off-by: Mark Menzynski <mmenzyns@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>