drm/nouveau/pmu/gm20x: don't pretend we support loading with our custom FW

It technically loads, and runs, but is ultimately pointless outside of
a very narrow window (fanless systems where one wants to attempt using
the, broken for a lot of gm20x, memory reclocking code).

It's also potentially dangerous to override the VBIOS-provided "Pre-OS"
PMU, which would be responsible for fan control otherwise.

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